Well first of all be sure he is "gone" as in dead. He could be hiding anywhere....I read your other post and sounds like he DID get killed, but lets say you go out tomorrow and pick up a leopard gecko then later that day out walks your moniter from some hiding spot. ????
I'd wait a week, then feel a bit better about purchasing something else. Lots can fit in a twenty gallon. Leopard geckos, I believe a Crested gecko would work out, Golden Geckos, um smaller snakes like Pueblan Milks, any young colubrid can live in that for a couple years....tons of stuff.