Updated version. 2.5mm arms and bottom plate. 

• -CAD picture below shows latest version.  Photos shows previous.
• -Stack height increaced.  Accomodates up to 23.5mm tall (from bottom plate to lower-end of top plate).
• -Stack space increased.  Accomodates up to 44 mm x 44 mm ESC.
• 16x16mm motors, naked-bottom only.
• -Smaller changes:
  • ----A few chamfers added for easier assembly.
  • ----Some unnecessary cutouts removed.
  • ----Cam plate should have slightly less freeplay
  • ----Proper orientation of top plate more easily identified (see CAD pic below)
  • ----20x20mm stack holes removed. (30x30 ONLY)
  • ----Bolt attachments to arms should be more rigid (thicker).  8mm screws are included with the design, but 10 mm would work best for bottom plate, 9 mm would work best for top plate.
  • ----Additional material added to front arms in drop-down portion for increased rigidity.

Similar frames are becoming popular with some German and Swede pilots for their flight characteristics (see Carbix frames or Blitzfpv frames), something to do with reduced drag and less drift due to the 8 vertical members.  Anyway, this is my take on it.  About 66g of carbon, the twelve 14 mm screws will certainly add a bit to that. Yes, that's a tilted back top-plate, mostly for looks.

These arms are just 2 mm thick (as are the Carbix frames), not much thickiness is required as the box-frame distributes impacts around the outside skeleton and jointly across all arms.  Hit my email up if you're interested in seeing a beefed up "tank" version, erikspen@gmail.com, that's an easy change to make in the CAD model, available for viewing here: Onshape model