As cool as polycarb is... I'm not sure I'd recommend using it for something like this - top panel could work but structural parts, HDPE outshines it massively from what I've seen personally... Steel for all those parts is definitely a good idea though. Get the angles right around the sides and it'll take very little damage from spinners!

I'd really recommend HDPE over Polycarbonate - Hatchet's baseplate was in quite a state after Mr Mangle hit it, cracked straight through... you could get some polycarb as armour easily but I think the trick is to leave it a little free so it has room to flex (like the arena walls have) so wouldn't be too useful for your structural components which would have to be quite tightly bolted. You can buy it quite readily online though, if that proves a useful option!