Catagories are: Ants - 150g, Beetles 1.5kg, Feathers 13.6kg

If you never plan to build 'true' Beetles and only go with acrylic/plastic chassis and limit what students can use for motors then I'd look at 5mm Poly for the walls and roof.

We built our own arena for Beetle Fights and Featherweight Demo's which is all 40x40x3mm Mild steel angle with 10mm polycarbonate walls and a 5mm Polycarbonate roof. At 2m x 2m its fairly big but you wouldn't want to go any smaller for BW combat.

Beetleweight arena at National STEM centre.jpg

That is the arena set up at the York STEM centre.