There isn't going to be a substrate that is going to even out your gradient.

You need to lower your warm side and warm up your cool side. A 55g is way too large for an adult ball python. Unless a snake needs a cage that large, it's really going to be extremely difficult to manage, near impossible to keep humidity, and expensive for heat.

If your set on keeping it, buy 2 red lamps put them on top for ambient heat. Buy 2 UTH's, and 2 thermostats.
Spray with warm water every day or buy a misting machine.

Use aspen, cypress mulch, or newspaper. No sand, pine, or cedar!