The nice thing about using radiant heat panels is that your cages will be suitable for just about about any snake should you choose to house a different species in some of those cages.
You are definitely doing it right.
The Bean Farm radiant heater they sell suited for 10-20gal tanks is a good choice for your cages. Plenty of heat. If you are unsure, buy one and try it first.
Another note just for your info: In larger cages, like 3-4ft cages, you can avoid high-temperature bulbs by using two or more low wattage bulbs. So in effect, using bulbs you can still get a warm cage without skin-damaging hot heaters. I just mention this just in case you ever have a need but don't have a radiant heat panel on hand. Radiant heat panels are still the best heaters though for most applications. You chose wisely.
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Mark