No wonder he's not eating...LOL -I would recommend using UTH, regulated down to about 86*...these are not snakes that bask, they prefer to burrow.
I don't have pics, but just picture using a cardboard box* like a hide that covers the whole cage floor, open to the bottom. And cut a few holes so he can come up from
"the basement". *I've made custom hides before too (waterproofed sides, tile board top)- cardboard won't last forever- & most other things that are moreless the right
shape (like a kitty litter box) won't fit in a 20L. If you manage to do this, the "upper floor" will be closer to the light also, making it warmer. (UTH is still best for milksn.)
*you can also use a large flat piece of corrugated cardboard cut to fit tank, only 6" longer & 6" wider than the tank- then cut the corners out & fold side flaps so it's about 3" tall. Does this help?Or did I confuse you even better? hahaha