The thing is, that is heating the entire tank, as soon as he comes above substrate then he's getting heated. Heat mats are only ever meant to cover half of the tank, be on the wall or on the side. Simply get a smaller heat mat, they're not expensive and doesn't have to be ET. It's better to get ones without the adhesive backing so you can reposition them at anytime and just use insulation tape around the edges to stick it on.

Plants do process waste but not as much as a Horned frog produces. People that do living vivariums use small species like Dart frogs or small tree frogs like Red Eyes which don't produce a lot of waste, therefore can be broke down easily. Most people keep Horned frogs in easy to clean set ups to prevent waste building up.