So what you are asking is - if you just stack some shoeboxes in some sort of open cabinet like in the picture and run Flexwatt down the back, will it work? If the room is warm, perhaps. But in general, no.
A better approach for simply stacking shoeboxes is this: make a large enclosure of plywood with a door with an acrylic window. Mount a light fixture inside on one side of the ceiling or make a screened cutout on top to one side. Use a red heat bulb or ceramic emitter and a proportional controller. Then stack your boxes inside this enclosure. Now you have a mini temperature-controlled snake room of sorts.
The enclosure mentioned above doesn't have to be sealed or built perfectly. It is just to help hold the heat in. On the cool end, make adequate vents. That way the shoeboxes will have a cooler side to them.
The method described above is how I handled keeping baby snakes before I got a real rack for them. It was easy and very effective. The real benefit was, for some species, you could move them closer to the heat source if needed (like for baby boas.) And the cooler-tolerant species go on the bottom of the stack. I like that aspect better than racks. So much so that I don't have the racks anymore. I can use any box I want that fits. No worrying about the dimensions changing.
Byron makes a great point. Some types of snakes need access warm temps for digestion, warmer than other snakes. His mention of rosies is fitting IMO.
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Mark