The idea of pushing it up against the tank with the reflectix sounds reasonable. Just make sure there is some ventilation as you don't want to fully insulate a heat source (fire risk, etc). Personally, I would just replace the mat. I have read horror stories of damaged UTHs causing house fires and killing animals. I may just be paranoid, but I would be losing sleep like crazy over the risk of an extreme temp fluctuation causing damage to the snake in the night or while I was away. I have read that the wiring inside these is very fragile and that they recommend not removing it and reusing it because it can be dangerous if the wires are compromised. I am not sure what size tank you are using but I have a 30 gallon UTH that I got on amazon for $20.00. If replacement is not possible for whatever reason, I would just recommend monitoring it extremely closely for a while. Keep in mind though that the cost of the reflectix roll alone would be about 20.00, so it really may be a comparable investment to just get a new heater.