I'm not sure how they will fare but I have just purchased a set of foam donuts which
I hope to try on my xxx-cr, years ago everyone used to use them at my local, I will be trying them in the next few weeks and will post my findings.
Not sure if all clubs allow them though so best find out first!
see link
http://www.rc-direct.co.uk/-c-39_198_205.html, one tyre makes one front and one rear so two donuts need to be purchased.