Did a quick arena to get started. Two layers of flooring underlay (smooth finish and cheap), some cheap plywood boxes between them to make the pits, some wood pieces to reinforce the edges, a little trim, and a couple cans of spray paint! About a 32" edge, with a 28.5" fighting area. Pain in the butt to transport, admittedly.
I used the Wilton 3" pillars to form the border, plasticard and some rocks to make a fighting ring, and some balsa strips to cover the pits when not in use.
The construction was easy enough, but with only a jigsaw to work with for cutting the edges aren't great and it isn't square. I won't build something like it again without a table and miter saw. It'll make things so much easier and I'm sure the results will be much improved.
Still need to fill the pits with dangerous stuff. I like the idea of a hot coal/fire, some spikes, a crocodile with water, and maybe snakes?