3 and 4 inch flexwatt is only good inside of a rack, it's not going to give you the ambient temps your looking for unless the room is 78 degrees or so.

I use 11X12 inch under all of my 28qt tubs and larger unless they are in an enclosed rack, then 3 or 4 inch works fine.

Holes do not go in the lid of the tub, they go in the side of the tub