I notice most racks, including the deep ones use side to side (width) heat tape, instead of using heat tape that goes in the center up the depth.

I would think the latter would allow the snakes to sit on more of the tape, instead of some being lost to the sides, it would also help prevent the tub from rubbing over the heat tape and would allow tighter fitting sides on the tub setup. Mind that I am not suggesting this run all the way down the tub's length, but instead sits under it running 1/3 to 1/2 the tub length, so a 3' tub would have 1-1.5' of belly heat tape running under the center of the tub when slid back.