i heard something a while back about a guy mounting them on hover boards make from 2 round bases stuck together, although that wasn't very imperium-esk. i've even seen someone model a bunch of cadians on rollerblades!?! funny as hell but again not very fluffy.
heres a link i just found with a google search for a rough rider conversion that should work out a little cheaper than the one on the gw site.
http://images.google.co.uk/imgres?im...%3Den%26sa%3DN