Ive done basically what Tsanford said with some success. I made my enclosure myself, so the way I fashioned it was to have the flexwatt run from inside the tank, under the supporting side walls, and then out to the periphery. I did this to allow my thermostat probe to be in direct contact with the heat tape, but not be in the actual enclosure. I built a false wall around the extruded heat tape and probe. Inside the enclosure I covered the flexwatt with fiber glass, and then used aquarium safe caulk to seal around the edges. I also actually drove a few screws through the acrylic into the melamine as well to hold it in place. I noticed that thicker acrylic works best. Even though the hot spot doesn't get past about 90, I noticed some bubbling when using thin fiber glass.

Honestly in the future I am not going to mess around with this. Seemed like too much potential for error for me. If there is a spilled water bowl, and for some reason the silicone caulk doesn't hold, your whole heating device can be fried. For the money, a RHP seems to work much easier.