The cables are meant to be buried in the potting mix and not laid on top of shelves.

I used a heating cable from Stokes several years ago. We built a heat table by making a framework of 2x4's, stapling hardware cloth to the bottom (to allow drainage), laying a sheet of landscape cloth in the bottom (to hold in the potting mix) then filling the "box" with several inches of ProMix, with the cable being buried in the mix.
The whole thing was on folding legs that are available at HD etc.

The table was setup in front of a large window and worked well for sticking cuttings for a couple months. Then suddenly the cable just quit working.....That was the end of that table.

I went to using heat mats and found them much more versatile. I have now been using mats for about 5 years. I have one 4 foot one and 3 single tray size ones and they have been reliable and useful.

You only want to put your seeds on heat for germination. Seedlings themselves prefer cooler temperatures and if left on heat will become elongated and weak.
In fact, different seeds have different temperature requirements and some aren't put on heat for germination, such as columbines, delphiniums, coneflowers. You will need to look up the requirements for each kind of plant you intend to start.

Thanks wyndyacre. I'm just trying my hand at begonia's and so far they are my first attempt at a plant which requires a specific temperature. The mat should work just fine and I'll leave the cables until I have a greenhouse.