the way i set up cages like that using heat tape, mind you it's 24x18x24 cages but.
I used 3 pieces of 12x12 heat tape wired together one top rear corner..
you can see the rear back on in this picture, then another one on the rear left side wall and top rear corner. This gives me a nice fully heated top corner, i think each 12" is 23W
so i get 69W of heating for 15$ versus a 100$ 40W heat pannel.
I only do this to save $$, heat pannel are way better but i must admit that this set up works extremely well..
so maybe getting some on the side as mentioned and maybe even the back would bump it up to where you want it. Some LED ligts do produce a little heat, adding some for daylight could help for daytime and then you would get a natural night drop that isn't to much when the lights go out and the heat tape keeps cozy..