Sharon,
One thing you can do is to simply put Flexwatt or Heat Wave heat pads under the back bottom of the cage. This way there is no drilling or wiring necessary. You can contact The Bean Farm - I believe they will wire up the Flexwatt for you to a power cord. You can get a 2ft strip of the 3" wide for each 24" cage and lay it right under the back edge of the cage. Those cages are very light in weight and will not hurt the heater or cage. You MUST use a dimmer though. The Bean Farm sells them, and so does Home Depot (look for the tabletop dimmer. No wiring necessary. Just get a 3-to-1 adapter so you can plug in three cords into one dimmer.)
Believe me- with snakes it is way easier to not have a heat pad inside the cage. So much easier to clean the cage without it.
Another method for you: Garters do not need much heat. Simply tape the Flexwatt or heat pads to the back outside wall of the cage. Put something (like fake rocks or a brick or a branch..) near the back wall so the snakes can cram themselves between the object and the warm wall. Works very well. This is how I kept my mountain kings.
The 3ft cage is way bigger than the 2ft cage. I housed two Ariz mtn kings that were nearly 4ft in one 3ft cage and had room to spare. You can easily keep 3 garters in the 36" cage. The 24" cages can house maybe 1 or 2 adult garters depending on the type.
Lastly, you can easily heat the 36" cage with a light fixture inside. The bulb will be high enough for the snakes not to climb on. But, you will have to wire it, so maybe you will want to bypass this method. I can give you detailed instructions on how to implement a ceramic socket inside a Neodesha (or Vision or Boaphile or ??) such that it will not burn, melt or harm the cage with prolonged use. A 50 watt red basking lamp is all you need to heat one of those cages for garters (and a dimmer of course!)
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Mark 