I am sorry but I think this is just wrong. If you read the warnings that come with the UTH it specificly says it is not supposed to get wet. Even with turf on top of it, it is still going to get wet. That would be like a human urinating on an iron that is plugged in and on! NOT a good idea. You said you have been doing it for years with no trouble... If my friend told me he was urinating on an iron that was plugged in and on for years without a shock, it is safe to say that i still would NOT be interested in trying it myself!
Another thought is that sometimes snakes get under the astroturf. If you put a zoo med heater outside the tank like your supposed to, the glass on the inside directly over the heater will still get up to 125 degrees! therefore you must use something that regulates the heat... I use a $6 lamp dimmer so the hottest it will get is 95 degrees. If your snake does get under the turf, he could potentialy end up sitting on 125 degree glass! ( the same as a heat rock)
You say you are using a 55 gallon tank. what are the dimensions of this tank? I have a 36" by 18" 30 gallon tank, and my UTH is 11'' x 17''. Assuming your tank is even longer and fatter than mine, you will have more luck with a bigger, more correctly sized UTH.
just my 2 cents
MIKE