MK3 bin box, squash feet edition

Yet another filament bin that goes in the usual corner thr the MK3 / MK3S/+, but since the previous Prusa MK3 bin couldn't be used anymore since I put the printer on 6 Squash ball feet I just redesigned one. Made it shorter since filament always falls near the back anyway, light weight and without annoying corners in the bottom, I went with 3 perimeters for a bit of rigidity.