I'm drawing a box for a small electronic project:
It measures about 200x110 mm and I'm going to print it with PLA. The first attempt was without the ribs you can see on the bottom and it was very easy to twist the box.
Adding them gave a perception of greater robustness, but I wonder if there is a more suitable shape to further increase the stiffness.
For maximum stiffness I recommend you make all the empty space solid (in the cad model).
Remember that 3D prints are hollow, so this will not increase the weight much. A reasonably thick shell with gyroid or triangular infill should give you better stiffness.
If you need it to be even stiffer, make the case thicker. If I wanted to make a plate with good stiffness in PLA, I would not make it less that 5mm thick.
When I design for stiffness, I like to imagine the part as a series of joints and trusses. If you do this, you'll see that a thin wall provides little torsional stiffness on it's own. The thicker it its the more the trusses have to deform for the wall to twist.
