For the arena to be 100% finished we need
4 sheets of 10mm polycarb 1975mm x 800mm
2 sheets of 10mm polycarb 1000mm x 2000mm
6 Strips of 3-5mm polycarb 50mm x 1000mm
After we get the polycarb we have to cut a doorway into 2 of the 1975mm x 800mm sheets.

I'm having trouble visualising this. Are the 4 sheets of 10mm polycarb 1975mm x 800mm the sides? If so, why does it need two doorways?

Are the 2 sheets of 10mm polycarb 1000mm x 2000mm fpr the roof? Given the cost of 10 mm polycarb why not consider making the roof of weldmesh with a secondary screen over it to catch any small bits?

A plain box with no internal arena seems the best approach to me; I've seen fights in a bare 8 foot by 8 foot box with this class and they were spectacular.