I have two suggestions for you. One, I'd suggest getting a digital thermometor - they are usually accurate off by one degree. You can pick up one at any electronic store, such as Radio Shack. A good option, but a more expensive route, would be to purchase a digital thermometer that incooperates the temp and humidity levels all in one unit; trust me it'll be worth it in the long run, and you'll have a probe which you can move around the cage and get different readings. The other option is a temp gun to measure your temps. www.tempgun.com has them. Those 'round thermometers' produced by ZooMed and other herp companies are worthless mainly because they are inaccurate. Think about it, is your frog basking on the side of the glass? No, its towards the bottom, underneath its subtrate, so how can a person really justify the correct temps? Just my .02 cents.
Second, if the temp is accurate, you can try sheet metal, and cover half the mesh wire top. Sheet metal is not flammable, therefore it is not a fire hazard and the heat and humidity won't escape as easily. Personally, if you're thinking about using a dome lamp, make sure the bulbs are not cermanic heat emitters because you could potencially dehydrate the animal and dry out the moisture/humidity inside the cage. Also, if it's the dome lamp route, you should use one or the other, and not a UTH AND bulb; too much heat in my opinion, unless the thermostat in your house drops below 66F.
Best of luck, and I hope you find a solution.
Have a good one.