Seal the board, with either hot glue or silicone, but don't glue it down. Simple.

That's what I'm doing with the cages I'm building right now. As for fire hazard, the ventilation holes in the wood, and a good thermostat, help prevent that. At any rate, only an actual spark would set the wood on fire as it would take a lot of just heat to do it...more than a UTH is capable of putting out.