I haven't read everyones replies here - so if this has been said I apologise.
I was in the tropics, Cairns, so heating wasn't usually an issue - however, I used a fish tank for my adult spotted python and she loved it. The tank I got was slightly defective, so not for water, so I got it cheap. As for heating, in the bottom, over my fake grass (I actually used marine carpet or shade cloth depending on my mood) I had bricks down the hot end. On the underneath of the glass I had a heat mat attached to a thermostat. Surprisingly it used to get a tad cold in Cairns from time to time.
The bricks would hold the heat for ages and the snake would bask on the bricks for long periods at a time.
The roof to the enclosure was Tasmanian oak frame with mouse wire - so it was completely open.
During the hottest months I had a set of 4 computer fans wired up to a 12V transformer that moved air efficiently out of the tank and kept the tank much cooler.
On the outside on the back wall and one of the side walls, I had the option of gluing some polystyrene - because it was on the outside of the tank it made no difference to the snake, no chemicals, no urine on it etc etc.
Plus I painted the polystyrene with water based paint, in blue...
Personally, even if I do say so myself, it looked awesome!
Unfortunately, when I moved I broke the polystyrene and I never replaced it.
Make sure with the heat mat you use the feet provided to lift the tank off the ground to allow some air flow.
This is the advice I can give to you from previous experience.
Good luck