I'm pretty sure collareds would be ok. Another good one to look at would be leopard geckos. Crested geckos would be ok in a 20 long but would probably prefer a 20 high. Also, anoles. And sand boas, can't forget sand boas.
-----
"The serpent crams itself with animal life that is often warm and vibrant, to prolong an existence in which we detect no joy and no emotion. It reveals the depth to which evolution can sink when it takes the downward path and strips animals to the irreducible minimum able to perpetuate a predatory life in its naked horror."
Alexander Skutch