I'm thinking now, my local pet store sells a 'cover' for a heatmat, which is supposed to make using heat mats safe in wooden vivs. If I could fit a probe under it as well, would that work? It would create a sort of 'step' in the viv, but that shouldn't be a problem, should it?
Edit: I'm now double thinking (Dangerous, I know) but maybe if I got some of that plastic sheeting (I'm not sure what it's called, like shower curtain material?) and hot glued that down over the heat mat to create a watertight seal, would that work? Hot glue would come off pretty easy I think if you pulled on it.