Give yourself a gap - also be forewarned substrate as easy to burrow in as aspen can allow your snake to snuggle its belly down flush against that hot glass. Belly-burns are far too common from what I have read. Another alternative is to make a heat baffle from trimmed PLASTIC pegboard (avoid the particle-board variety liek the plague) - if cut to the inside dimensions it can be laid on the bottom of the tank and substrate applied over it - hell with that setup you can just plop down some paper towels. If the board leaves too much room on the perimeter the snake WILL work its way underneath. Lemme see if I can find some pics of that stuff.