So my original design has some serious flaws. Going back to the drawing board, I decided to start completely from scratch.
I had two problems. The first was that all of the screws were not as accessible as I needed them to be. The second was that the fram required me to drill holes into the short ends of the front and back walls. This would have required a manual step and I could never get good enough tolerances on that.
Besides, anyone can design a machine that is hard to build. Making a machine that can be manufactured using a minimum of steps and realignments is the challenge. So what I wanted was a design where the motor mountings were the only pieces that required milling from more than 2 sides.
So here is where the magic happens. By using the motor mounts for corner support I think that the design will be completely solid. By adding a few extra interlocking edges as well the screws will not be the only thing keeping the pieces together meaning absolutely nothing should move.
Speaking of movement the side walls should be able to take a load of 25kg without buckling. Aluminium really is a fantastically strong and light material. I see why they build aeroplanes from it. We are after all talking about a structure mostly made from bars that are at most 4mm thick in either direction, most are 2mm thick in at least one direction.
To top it off, here is a rendering of the entire design.
