I'm using homemade cages, I'll take some pics. Mostly 2x4's and plexi.
That makes your situation more difficult as I don't believe those cages will be the most efficient.
What is reflectix and where can I get it?
Reflectix is one of many types of insulation that looks like bubble-wrap covered with aluminum foil. It is a potent blocker of radiant heat.
Here is a picture of my last garage snake room that I temporarily insulated with Reflectix.
I bought a roll at Menards but I have seen it for sale by the foot at Ace Hardware.

What are radiant heat panels?
Expensive yet efficient panels that can me mounted inside of cages and produce a heat that really does well with snakes, especially the heavier bodied species you keep.
Where can I get those and how much?
You can get them at: http://www.pro-products.com/
They are expensive, but given your needs I believe you need to spend money somewhere.
BTW... Do you know how to get the photo uploader to work?
I have never had any trouble with it so I've never had to play with it to get things to work. Sorry.
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Current snakes:
0.1 Gonyosoma oxycephala - (Silver/Yellow)
1.2 Gonyosoma oxycephala - (Green)
7.6 Gonyosoma janseni - (Black)
0.1 Gonyosoma janseni - (Black & Tan)