alot of people do a sand, topsoil, vermaculite mix i beleive..
ive used sand and topsoil before with good results. the top dries out and is desert like in appearance but it stays damp below, which allows them to stay hydrated real good. this also makes it so you dont have to add any other form of water into the cage. so you dont have fungus problems in the cage.
this also allows you a good solid place to but objects to climb on and suck to use as basking sites. or you can just use a rock. the basking site needs to be HOT i mean like 120* hot surface temp.. ill see if i can dig up pics of some of my old uros to show you.. im a big fan of high yellow male malis, as well as high black ones. which is what i will be going with in my next uro tank.. hope this helps
Mike