So far for me the best way is to just have the cage (plastic bin) rest right on top of the heat tape. Now several things need to be known about the heat tape first. Heating elements like heat tape use too small of conductors and alot of electricity, this creates heat. See the black vertical lines? They are really thing strips of metal. When you push a bunch of electricity through a small wire, it heats up. Along the sides of the heat tape is the main supply of power, a copper strip. When you put the tub right on top of the heat tape, year after year, pulling the tub in and out, it will ware down the top plastic protecting these small black wires. The best idea is to put some foil tape over the area that will get the most rubbing from the plastic bin. In the more expensive racks, they will put a thin strip of metal over the heat tape. This can be costly, but has two major benifits, one it protects the heat tape. Two, it distributes the heat evenly. This is important as heat tape will not always evenly heat, and if you put your thirmostat on the tape, you could have it on a hot/cool spot and het bad heating. Why I dont use a metal sheet over mine is because I want to be able to inspect the heat tape at all times. I dont want to have to tear apart my rack to get to it. Thats why you see it attached to a board, that fits in a pocket. If anything happens to it, or its just not needed, it can be removed with out effecting all the other cages. Most important, make sure all possable metal edges get covered with several layers of electrical tape, and all 4 edges of the heat tape get taped down, so nothing can catch on it. If you bend that heat tape and ware the wires thinner, it will get hotter (going back to the whole too much electricity through a small wire thing) and before it has a chance to break and just not work, it could melt the plastic and catch something on fire.
Heat Tape is NOT UL rated. This means they have no safty rating. Now who knows why, I do know its expensive to have done, and would make the product more expensive. When I make PC boards for things, UL rating STARTS at $1000, after I have paid $250 for an electrical eng to look it over. That all said, its a use at your own risk product. Thats why all the products you buy off the shelves in pet stores are so bad at heating, they have to tone them down so much for them to pass the UL rating. Who knows how hot rocks ever passed (it they did).
Ben
American Tropics