Glass tanks with screen tops are really difficult for maintaining heat and humidity for reptiles.
I think you are on the right track with the insulation. Cool glass not only cools the air temperature but it also sucks humidity via condensation. You might step up the insulation on the three sides with some of the Styrofoam sheet insulation (be careful not to create a fire hazard) and the insulation on top with tight fitting Plexiglas. I've purchased both at Home Depot. You can cut either to size with a utility knife or if you have good measurements you can have them cut the Plexiglas for you with their glass cutting equipment. Drilling a few air holes in the Plexiglas without making ugly cracks around the holes was a little difficult for me (I ended up clamping it tightly between two pieces of wood) but perhaps with the right drill bit it would have gone better.
You might well still find that some type of radiant heater from above like the ceramic bulbs is a good idea. Perhaps you could shine it through a side so as to avoid the Plexiglas and also stay away from the strophe insulation.