Thank you so much for all the help. Rudy is about 4' long BTW.
I am going to set him up in the 20 gal tank tomorrow and see if I can keep him warm that way.
I did not realize that the pads would not increase the ambient temp and I put it on the back side of the tank so he would not burn himself on it. The one under the tank is covered by his water dish so he cannot lie directly on it.
I do have a thermostat and I will use it when I switch tanks.
I will try some of the other suggestions like the reflecting tape if I find that it is still too cool in there.
I'll keep you posted on my progress.