First thing, high temps can cause neurological damage before they actually kill a snake, so if I were you, I'd keep my snake in the former (too small) set-up until such time as you've worked out all the temperature bugs on this 20L for at least a week for safety! Cramped quarters aren't near as lethal as wrong temperatures can be.And no matter how much you want your snake in this 20L, you'll be stressing them out & maybe causing food refusals if they stay there, even if you keep the temps. safe from now on.
90* should be the highest temperature that's any place your snake can touch, including under the substrate where the UTH is, or over the hide, or whatever. And you never heat the whole enclosure/tank/tub- your snake needs the unheated part for safety (to get away from excessive warmth & to thermoregulate), & it should be 75-80*.