I searched and searched and ended up making mine out of 1/8th foam donuts, a tyre truer, some sandpaper and some off road rims + strong glue. VERY messy! But the buggy was on rails. I ran them on a ZX5 so hex fitment front and rear - Still have them lying in my cupboard!
There are other threads on this, here's mine:
http://www.oople.com/forums/showthread.php?t=32648
Dan
