Something I'm thinking that might work better until you can get a different enclosure is this. Get a roll of either aluminum foil or wax cooking paper, a roll of aluminum tape, and a piece of fiber board, cardboard, or cork board.
-Remove the screen top, cut a piece of foil or wax paper to about 14" X 14". This will go on the inside of the screen to keep the snake from rubbing on it. Fold the edges around the edges of the screen and tape it to the outside of the screen. You will probably have to cut slits for the screen clips to fit through.
-Cut a piece of whichever type of board to fit over the top of the screen and tape it down. This will help insulate that end.
-Lay the tank on it's side with the hinged doors on top.
By doing this, you tank is now aproximately the same as a 20L tank. It will give you more floor space to provide a better temp gradient. You may have to get either another UTH or one that will cover approx. 1/3 to 1/2 of the floor space. Since you are using a dimmer, I would say monitor the temp of the glass where the UTH is and keep it around 91-92F. If you are using Aspen I would say about one inch thick would be good. As the snake shuffles the substrate or packs it around the temps should be good. If you are using paper towel or newspaper then 2-3 layers would be ok...