the only heat pad i would trust to put inside a tank or tub would be the Kane heat mats. they're rugged and water proof.

side heat and/or back heat is fine. some heat racks are designed with this built in already. it will provide the hot spot but your ambient temps should probably still be monitored and controlled by your house or room thermostat.

otherwise the tub bottoms grids not having direct contact with UTH is still fine as long as u can reach the optimal temps for a hot spot. again some racks have channels machined into the floors of each shelf for heat tape and tubs don't exactly touch them. i have 2 racks like this and still get 90degrees for my BPs.

so there's different ways to get the correct husbandry. u can try one with side and/or back heat and the other with belly heat and see which works best for your snakes. just some of my thoughts. hope others will chime in as well.