I think only toughened glass would be acceptable due to safety concerns. But perhaps using a fishtank or lizard/snake tank frame would be nice option and then retrofitting polycarbonate in place of the glass. Personally I think any arena must be large enough for the robots to accelerate and allow room for a chase. For ants a square 1.5m X 1.5m would be a minimum I think. Any less and it would be like two sumo wrestlers in Willy Wonka's glass lift.

On a larger scale I what just came to mind when I was writing the above, the competition squash courts they use of televised matches. Polycarbonate three sides I am sure. They would fit featherweights. Surely they would be way to expensive and out of scope. But if I see see one on eBay...... lol