If i were you Tom - knock the roof down to 5mm. like you say 10mm is OTT even for the outer screens, but if your running feathers for testing then 10mm is safe. Any form of meshing/netting/fabric roofing has been proven to by insufficient for any form of arena protection sadly. You would be amased how effective just 3mm polycarbonate is in comparision to other materials such as high quality plywood.

Just FYI, 3mm polycarbonate is about equal to 18mm plywood in terms of stopping projectiles (if a large enough sheet is used then much better than 18mm ply).

Realistically, to be safe for the kilobot weight limit i would recomend about 4mm sides and 3mm roof as a realistic thickness in polycarb if you werent testing feathers.

Also, if your having a battlebot's style arena, it's still worth having a reasonable distance from your inner walls to your outer screens (even if by a raised platform to drive back in) as it will stop your oter screens getting hit all the time and therfore wont get scratched and damaged so easily.