I've been using cork, balsa, plasticard, cast bases, Squadron Green putty, Skullcrafts Steampunk basing kit, and most recently, two sizes of Woodsies craft circles from Michael's. One's almost 30MM insert size, the other almost 40. I center the smaller one, and it looks fine. The larger ones need a few trips around the Dremel, or heavy sanding to take off about 1MM.I have Sculpey and greenstuff, but no time to use them properly.
I used Guardsman wood stain touch up markers (About $5 for light, medium and dark in a 3-pack at WalMart), then Vallejo gloss, or just use a matte spray coat.