I don't know for sure but I can't help but feel that having the whole floor such a hard surface that it might cause health problems with their toes, feet. Seems like I've read about problems with beardies and/or monitors under similar conditions. Might be something to look into to make sure.
You've got a good idea with using a thin layer of sand to fill in voids for good heat transfer. However, I seriously doubt that a UTH would be very effective at all in heating something 1-inch thick or more. Plus, the weight of anything that thick would be quite a bit.
What about going to Home Depot, buy ceramic tile, and have them cut it to where you'll have 2 equal-size pieces to cover the bottom with. Have them cut it so you'll end up with about a 1/4" gap between the two pieces (in the middle of the tank) and you can fill that in with play sand. You can also fill in around the edges, too, helping to better secure the tile. You can get the tile in 1/4", 3/8" or 1/2" thickness. Any of those will work nicely. I'd be surprised if it would cost more than $20.
You can even go to carpet/tile stores and ask if they have any old samples or tile left over from jobs and they can even cut it for you. $10, and you're out the door. Gone in a jiffy.
I use a big piece of ceramic tile in my savannah monitor's cage & my nile's cage. Their basking rocks are on top of the tile. The rest of the cage is dirt.
Just a thought in an effort to try to help you out. 
Take care!
HH
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