https://www.lowes.com/pd/PLASKOLITE-T-x-W-x-L-White-Corrugated-Plastic-Sheet/3381288
this is a 4mm corrugated plastic like what is needed for the side panels (it uses 4mm for sides, 3mm for the front plexi; 0.157″ = 3.9878mm, whereas 0.125″ = 3.175mm).
If you modified the designs on the enclosure parts, you could use hardboard, which is about 3mm (0.115-0.125).
