[cbale2000]

Don't get me wrong, I don't have a problem with using polycarbonate (we've used it extensively throughout our chassis for years) but it looks like you're using maybe 1/8" at the most?

On our drive system we use 3/8" polycarbonate for durability. Granted we're not bending flanges into it, but I personally don't think I would ever use less than 1/4" thick on a chassis. Polycarbonate is roughly 44% the weight of 6061 aluminum so you do still save weight even with twice the material thickness compared to a typical aluminum frame.
Additionally, if you use thicker polycarbonate, you could potentially replace even more of the aluminum frame with polycarbonate like the interior plates that you mounted the gearboxes to. I think I would still leave the front and back parts aluminum though for rigidity.

The other trick we've found is it has to be supported with standoffs and such otherwise you get too much flex between attachment points (as it seems you experienced).