You could buy a low wattage heat cable/cord and experiment with that. They are incredibly adaptable. You could wrap it around a limb made to hang above his head, you could attach it to the side or back of your tank, you could even just lay one across the top of the tank to see if the heat goes far enough down to benefit your frog. What if you attached it to the bottom side of your lid? Then the heat would be trapped in the enclosure, but the source of the heat would be above head, and burrowing would still provide relief if he's trying to escape the heat.
I have a few heat cables at home, and I honestly dunno what I'd do without them.
Zoo Med 11.5 foot Repti Heat Cable