You can place the flexwatt on top of the wood. Make sure you use a thermostat, or at least a rheostat unit to make sure the heat build up between the flexwatt and the cage bottom doesnt get too much. You are looking at achieving a hot spot of around the low to upper 80s depending on what species you are keeping. Not cook your snakes!!
I know a few breeders and keepers of sand and rosy boas who use rack systems. Smaller males of both can probably stay in smaller box sizes, like 15 quart. The larger females may need the larger 28qt size, but only if three feet or longer IMO. Also, since the boxes are plastic, make sure extra ventilation holes are drilled or melted along the sides of the boxes for rosy and sand boas. They like it drier in general and the typical rack system only have a small gap between the top edge of the box and the overlying shelf. This may not be adequate for those two desert boas.