Core's transponders are paper thin, but will wear after a while since they are not much more than paper. There are also various types of Amb-It transponders--the personal transponders for RC are smaller and lighter than the loaners, which need to be charged. I also removed most of the plastic from my personal AMB-It so I could fit it inside my non-F1 Kyosho Mini-Zs. The last I knew, the Ko Propo system was more limited as is the LapZ system. Here are a few links if anyone is interested.

http://www.corespeedway.com
http://www.amb-it.com
http://www.kopropo.com/arc.htm
http://www.lapz.com