This has features that you cannot have with injection molding.  has drainage channels for superior drainage of water from the bottom of the 4x4/100mmx100mm.   Drainage channels can also be used to tie string for lifting in and out of dug holes.

Not sure if this will keep my fence posts from the bottoms rotting out, but I hope this works.

Recommend:

1. three perimeter walls (thx to CNC Kitchen)
2. high % of infill if not solid (will be under a lot of compression)
3. 4x4 for 200x200 bed
4. 8x8 for 300x300 bed

*100% infill for 4x4 was only 150G/50m @ 15hrs

**I am not a real engineer just simple mechanic, seen some sh1t break.