I asked someone in the US who runs an arena for Beatleweights what wall thickness he used for his. He said that damage to the walls from weapons was a problem and so he used an insert with 20 cm high sides and allowed 15 cm clearance between it and the sides. He uses 6 mm polycarbonate for the sides and roof and doubles it up for the bottom 30 cm of the walls to allow for machines that get thrown OOTA but are still runing. His arena is a bit bigger than the one being discussed here but I think you could get a 180 x 180 cm insert into the one being discussed here and still leave 10 cm clearance around the insert edges. A picture of the US arena can be seen here: http://www.calbugs.com/CIB%20Arena.jpg