The front standoff mounting holes are suitable for using "Diatone ROMA F1 Camera Metal Protector" for a Nano size camera. For that purpose, the rear standoff height should be 20mm.

https://www.diatone.us/collections/roma-acc/products/diatone-roma?variant=39321176637527

The following picture shows my 3" light weight build with Diatone ROMA F1 Camera Protector.

This is the Bat-Hex kit with 2.5mm Bottom Plate (carbon only).

• Please note that the mid arms and braces may be not robust enouph for had crash. 
• FC, ESC, VTX mounting holes: 25.5x25.5mm(Front only), 20x20mm, 16x16 (front and rear), M2
• Motor mounting holes: 9x9mm, M2
• Standoff mounting holes spacing: Front=18mm, Rear=14mm; M2
• Prop size: 2"(flat), 2.5"-3"(front and rear props should be pusher)
• Thickness: bottom=2.5mm, top=1.5mm
• Weight: About 21g
• Betaflight Motors Mixer: HEX X

The front standoff mounting holes are suitable for using "Diatone ROMA F1 Camera Metal Protector" for a Nano size camera. For that purpose, the rear standoff height should be 20mm.

https://www.diatone.us/collections/roma-acc/products/diatone-roma?variant=39321176637527

The following picture shows my 3" light weight build with Diatone ROMA F1 Camera Protector. This frame is a prototype and is not the same as the product.

This is the Bat-Hex kit with 3mm Bottom Plate (carbon only).

• Please note that the mid arms and braces may be not robust enouph for had crash. 
• FC, ESC, VTX mounting holes: 25.5x25.5mm(Front only), 20x20mm, 16x16 (front and rear), M2
• Motor mounting holes: 9x9mm, M2
• Standoff mounting holes spacing: Front=18mm, Rear=14mm; M2
• Prop size: 2"(flat), 2.5"-3"(front and rear props should be pusher)
• Thickness: bottom=3mm, top=1.5mm
• Weight: About 24g
• Betaflight Motors Mixer: HEX X

The front standoff mounting holes are suitable for using "Diatone ROMA F1 Camera Metal Protector" for a Nano size camera. For that purpose, the rear standoff height should be 20mm.

https://www.diatone.us/collections/roma-acc/products/diatone-roma?variant=39321176637527

The following picture shows my 3" light weight build with Diatone ROMA F1 Camera Protector. This frame is a prototype and is not the same as the product.

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The front standoff mounting holes are suitable for using "Diatone ROMA F1 Camera Metal Protector" for a Nano size camera. For that purpose, the rear standoff height should be 20mm.

https://www.diatone.us/collections/roma-acc/products/diatone-roma?variant=39321176637527

The following picture shows my 3" light weight build of the 2.5mm bottom-plate version with Diatone ROMA F1 Camera Protector.

The most AIO FC (F411, F405, ...) can controle six motors if you make resource remapping the LED_STRIP, unused UARTs, or unused SCL (I2C) to M5 and M6, and make those pins enable timer and DMA (for DSHOT). So, I mada a kit which consists of a usual toothpick frame and an additional arm and spacers whch extend a quad toothpick to a hexacopter.

An addional mid arm has 130mm Motor-to-Motor length, which is enough for 3" hexacopter.  Using motor mount spacers (5mm), you can align the directions of all propellers of thickness less than 7.5mm. For a successfull build, two additional motors should have diameter less than 18mm, othersize 3" propellers on the front and the rear motors hit the mid motors.

If you want to mount additional ESCs under the main frame, the Minimal Bottom Plate 1.5mm is reccomended for protecting them.

• FC and ESC mount : 16x16mm, 20x20mm, 25.5x25.5mm, M2
• Compatible with the whoop-type USB connector
• Motor Mount : φ9mm M2
• Mid Arm Legth: 130mm Motor-to-Motor
• Motor Mount Spacer Thickness: 5mm
• Compatible Motor Bell Diameter: Less than 18mm
• Weight : About 12g

This kit consists of the SimplePick-EX 3mm version and four of Arm-Splitter (2.5mm). If you have an AIO FC with F405 or above, it is easy to build an octocopter even if it does not have enough dedicated motor pads. For that purpose, you only need to make resource remapping the LED_Strip, unused UARTs, or unused SCL (I2C) to M5-M8, and make those pins enable timer and DMA (for DSHOT). There are 1-2S AIO FCs with F405 by JHEMCU and Flywoo, it is also possible to build a 1S octocopter.

If you want to mount an additional 4-in-1 ESC under the main plate, the Minimal Bottom Plate 1.5mm is reccomended for protecting it.

• FC & ESC Mount : 16x16mm, 20x20mm, 25.5x25.5mm, M2
• Compatible with the Whoop-type USB connector
• Motor Mount : φ9mm, M2
• Weight : About 16g

This kit is designed for a 3" toothpick style X8 octocopter. 

TuneRC Poly F405 2S-4S AIO FC has soldering pads dedicated for M5-M8. Now, anyone can build a toothpick style 3" X8 by adding a 4-in-1 ESC and four motors with this kit.  Even if your AIO FC (F405 or above) does not have soldering pads dedicated for M5-M8, you  can still make an X8 by making resourse remapping LED_STRIP, unused UARTS, or unused SCL (I2C) to M5-M8, and making those pins enable timer and DMA (for DSHOT).

If you want to mount an additional 4-in-1 ESC under the main plate, the Minimal Bottom Plate 1.5mm is reccomended for protecting it.

• FC and ESC Mount : 16x16mm, 20x20mm, 25.5x25.5mm, M2
• Wheel Base : 140mm
• Thickness : Main Plate 3mm, Motor Mount 2.5mm
• Weight : About 16g

As for compatibility with the FPV Cage series, this frame kit with 3" props is compatible only with the FPV Cage for 14mm Cam. 

• Optimized for Diatone MXC 25/L3/L4/TAYCAN  C25 Camera Protector and 23mm rear standoffs,  M2
• Camera Mount Width : 20mm
• VTX Mount : 16x16mm, 20x20mm, M2
• Main Frame Mount : 25.5x25.5mm, M2, BetaFPV Clamping Nuts
• Thickness : 1.5mm
• Weight : 11.6g (including hardwares)

Diatone MXC 25/L3/L4/TAYCAN  C25 Camera Protector
https://www.aliexpress.com/item/1005001307694764.html

BetaFPV M2 Clamping Nuts
https://betafpv.com/collections/cable-screw/products/m2-clamping-nuts
https://www.aliexpress.com/item/4001197769908.html

• Optimized for Diatone MXC 25/L3/L4/TAYCAN  C25 Camera Protector and 23mm rear standoffs,  M2
• Camera Mount Width : 19mm
• VTX Mount : 16x16mm, 20x20mm, M2
• Main Frame Mount : 25.5x25.5mm, M2, BetaFPV Clamping Nuts
• Thickness : 1.5mm
• Weight : 11.5g (including hardwares)

Diatone MXC 25/L3/L4/TAYCAN  C25 Camera Protector
https://www.aliexpress.com/item/1005001307694764.html

BetaFPV M2 Clamping Nuts
https://betafpv.com/collections/cable-screw/products/m2-clamping-nuts
https://www.aliexpress.com/item/4001197769908.html

• Optimized for Diatone F1 Camera Protector and 20mm rear standoffs,  M2
• Camera Mount Width : 14mm
• VTX Mount : 16x16mm, 20x20mm, M2
• Main Frame Mount : 25.5x25.5mm, M2, BetaFPV Clamping Nuts
• Thickness : 1.5mm
• Weight : 9.7g (including hardwares)

Diatone F1 Camera Protector
https://www.aliexpress.com/item/1005002833101286.html

BetaFPV M2 Clamping Nuts
https://betafpv.com/collections/cable-screw/products/m2-clamping-nuts
https://www.aliexpress.com/item/4001197769908.html