Some related designs Designed to fasten 2020 Aluminum Extrusions to make cubes with flush edges. Not intended to be particularly high load capacity, but I've built quite a few frames from this general design, and they are working just fine for my needs! Holes are designed for M5x16 BHCS fasteners, but slightly shorter (maybe down to 12mm...maybe...?) should work.

Mine have printed fine without supports, brims, etc.