I use a plastic shelving unit like reptile frenzy said. I tape the heat tape directly onto the shelf. Ideally there'd be a barrier so the tub doesn't rub the heat tape. I personally don't do that but I also constantly check my heat tapes wear and tear. And no it shouldn't hurt the temps/set up if it's slightly too big. How much it hangs over would affect the stability, if it feels anything less than extremely solid I wouldn't risk it. But if it hangs just a little I don't think that wouldn't really matter. Here's a couple pics of my set up