Something like 2m x 2m arena, 0.75m to 1m high, 3-4mm mild steel floor or better: Hardox, 6-10mm polycarb on all sides, 2 lockable doors. If you go with 6mm polycarb then you may find yourself replacing it after a while, 10mm is what is used for the FC feathers. Frame should be ok with 20x20 or 30x30 box with a 3-4mm wall thickness.

It all depends on what you want from it. We plan to test our feathers in it so we have over spec'd the polycarb and the frame, but its also easy to disassemble which has added to the weight.