well that narrows it right down.you are going to have to have a species that lives in a dry enviroment as the moisture of a humid enviroment would rot the wood.Then because its a heatmat that def rules Rankins out (smaller version of a beardie) because they need a basking spot bulb.Are you able to put a spot bulb and UV lighting in the viv?If not i would say Leopard geckos as most of the bigger lizards need UV lighting.
Or you could keep a snake or 2