There's a forum for cage design/construction, but...
Just make sure the contacts to the heat tape are very well insulated before you put any kind of foil on top. Even if you get those little plastic clips, I'd solder the connection for the sake of it, and wrap the whole thing in electrical tape before the foil. Last thing you want is something shorting out/shocking something.
It does work, though, pretty well. I have 1' chunks of 3" heat tape on top of a 14" X 5" slab of plexiglass wrapped in foil tape. It distributes given enough time, and I can move these "plates" around the cages as the weather changes.
You might want to initially check how much heat you get out of whatever you build, and maybe put on a rheostat in addition to the thermostat. You would rather have slower, steady heat than high, strong heat in little bursts of time, especially because it takes time for heat tape to cool off/heat up.