you can do that, just make sure you monitor the heat with a thermostat or at the very least a dimmer and have it set so it never gets above 95ish degrees, or else the glass might get too hot and if the snake decides it wants to climb the glass on that spot, it might burn the snake. UTHs are known to get well over 120 if left without a thermostat or dimmer.
Also, unless you've already done so, try covering a portion of the top with plastic. It'll help to keep heat from escaping.