Dangers of UTH on SIDE WALL of tank?

[rosequoll]

I've been searching around online for a few hours and haven't been able to find exactly what I need to know, and while this isn't a DARTfrog tank, it is a frog tank so I'm sure you all can help. =)

I've created a planted tank using LECA, egg crate, flyscreen mesh and substrate. With plants (I'll attach a photo). When I first set up the tank it was simple LECA and spag moss with a UTH, which was keeping the tempteratures fine but 'burning off' the water in the LECA very quickly.

Since creating the egg crate bottom raised up, I'm now not getting very high temps in the tank. It's currently fine, but it's only heating about 1.5-2 degrees above ambient temperature and while it's 26`c/79f out that's fine, but once winter kicks in soon (already getting chillier at night, I'm in Australia so we're heading INTO winter now) I'm worried that I'll come out to 14`c/57f frogs. =(

I've added a bit of water into the LECA area hoping that it will heat up and help, but I think perhaps a heat mat on the SIDE of the tank may help the matter a bit more. I have a few worries here though:

1. I created the raised eggcrate bottom because I was, quite literally COOKING the plants, and the spagmoss bottom was easily reaching 32c/89f or more just below the surface. I don't want to create too much heat and fry the plants again.

2. I have tree frogs, and I'm quite worried about them burning their bellies on the glass if the heat mat is too warm. I can add little rubber feet/spacers but I don't want to even RISK this happening.

3. I'd go with a heat lamp instead, but this will burn off the humidity far too fast and bake the plants (and frogs).

Thanks for any help you can give!