Right idea for DIY rack?

I'm looking at building my own rack, let me know what you think:

I'll use melamine, with 3 shelves for 3 tubs, and 11" flexwatt (20 watt/ft) as backheat, controlled by a thermostat. The melamine will cover all sides but the front, with the flexwatt attached to the back.
This will be for sub-adult/hatchling ball pythons, but I'd like for it to be used until they are adults. Should I go with 40qt or 28qt tubs?

Should this setup work fine?

Also, what are the positives/negatives of using back heat/under heating

For using the bigger tubs I would personally go with belly heat. My baby rack with 6qt tubs has back heat and I have to have the to set pretty high to get the correct ambient temp. Using a bigger tub would require even more back heat to get good ambient heat throughout the tub.

Belly heat is only bad for hatchlings because of the size of tubs used.
It's more difficult to achieve the proper temperature gradient in the small 6 quart hatchling tub, so that is why back heat is preferred there. With belly heat, heating up a 6 qt tub can mean just a hot spot and no 'cool side'.
If you were to use a larger tub (i.e. 12-15qts), using belly heat is possible. But I believe the 6 qt tubs can be used if you don't put the tub over the flexwatt all the way. It really depends.

Personally, I use belly heat for my sub adult/adult snakes (28qt tubs). I really like back heat, but it didn't work out well in my house.
I have a room temp of 70-75 degrees. When using back heat, the flexwatt was 120 degrees to achieve a 90 degree hot spot in the tubs. That's because back heat heats up the air rather than creates a specific hot spot like belly heat.
Flexwatt isn't designed to run over 100 degrees so I was rather uncomfortable with it running 120. I switched over to belly heat and in the same room, the flexwatt only had to run about 98 to achieve a 90 degree hot spot.

It really depends on how warm your room temperature is. If your room is 80+, back heat works well. Anything lower, it doesn't work very efficiently or safely.