Chad,
That many 4' shelves will be very difficult to heat with 23' of cable. That's a guess on my part, though, not having any specific experience with the cable.
Why don't you buy some boxes and test the product a bit before you decide?
Really all you need to do is nest the cable between some 1/4" thick strips of wood and set your boxes directly on top. Take temperatures all over the floor of the box.
If it's not warm enough, or conversely, the heated area is too hot to achieve warm areas at the cooler end, then just loop another coil under the tubs.
If you find that you need two loops per box, for example, then you should be able to get away with three levels.
If it's not as efficient as you want you'll have to consider other products.
Otherwise, I really like the plans for the hatchling rack at ARBreptiles.com -- I'll include a link below.
You may have to modify the dimensions depending on the size of your boxes. Also, you'll have to include a groove or some shims for the cable.
Since leopard geckos are not the greatest escape artists you can have pretty nice gap between the box and shelf above. Trust me, better too big of a gap rather than too small.
ARBreptiles.com hatchling rack
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Current snakes:
1.1 Gonyosoma oxycephala - (Silver/Yellow)
3.4 Gonyosoma oxycephala - (Green)
2.1 Gonyosoma janseni - (Black)