Here's the thing: if you can hold the ambient temp at a nice cooler range like 60-75, then it becomes easy. Use Flexwatt or whatever.
But when the ambient air temp changes seasonally, a wide heater that is great in Winter becomes too much heated floor space in Summer.
My thought is to use two small heat pads per box or two independent heat tapes each row of boxes. This way, when you need more heated area, use both. During warmer weather, use only one. A nice thing about kingsnakes is that they can warm up on a thin strip of heater.
Here are some pics of ultra thin heat pads and really nice 3/4 inch wide heat tape. Not to say you should use these items persay. I think the ZooMed heat cable or the 3inch Flexwatt is alot cheaper. But the silicon coated heaters in the pics are awesome. I am slowly replacing any heat pads with these ultra thin ones, as they work so well for plastic or glass cages. They are easy to tape down and conform to the cage bottom better than mylar coated heaters.


-----
Mark