You your confusing a thermostat with a thermometer.
A uth hooks up to the thermostat and when the heat pad reaches the set temp on the thermostat it turns the thermostat off. This is to keep the pad from getting to hot and burning your snake. The zoomed pads easily get 120* thats is too hot.
I would say go with a 20g.
And get the acu rite digital thermometer/hygrometer
BTW you dont need a normal light. The light from the room is good enough.
Wood thing is up 2 u. One more thing u gotta clean
As for hides..Cereal bowls with a entrance cut out, kitty litter pans, terracotta pots. Anything u can tip upside down he can slither into and feel it touch all his sides
Also as mentioned before u could always leave him in it if u crowed it up. But thats up to you