For Hive I bought some foam rounds from these guys, http://www.efoam.co.uk/

Cut them to fit around the rim and then rammed it in hard. Worked like a treat with punctures to the rims (shame the rest of the bot had issues). It was a recycled foam made from small bits of different foams glued together (cheapest they had). They did weigh a fair bit when all was said and done though.