These are carefully optimized to bring as much rigidity as possible at a given size. They are of course still plastic, but I have a bunch of these in my HyperCube and they are indeed very rigid. The 20mm ones are drop in replacements for aluminium ones, but print larger versions if you have the room as they are stronger.
- 20mm, 2 screws
- 25mm, 2 screws
- 30mm, 2 screws
- 40mm, 4 screws
- Bonus bracket which fits over your aluminium ones
Walls: 3-6, higher gives more rigidity
Edit: I made a new, customizable version of these. If these aren't the specific sizes you want, check it out: https://www.thingiverse.com/thing:3007147