Without knowing for sure, I'm going to guess you have about 40 Watts of Flexwatt per cage (2ft of the 20Watts/ft material per cage). For 5 cages, that is 200 Watts. Since I don't know for sure, I'll play it very safe and say you have 300 Watts of heater load.
For now, you may get two plug-in dimmers (called "table-top" dimmers). They are about $11 each and are sold anywhere lamp dimmers are sold.
You will need two of those 1-plug-to-3-outlet thingies, you know, the inexpensive molded plastic thing that you plug into an outlet and it provides three 2-prong outlets.
1. Plug dimmer into wall outlet.
2. Plug a 1-to-3 outlet thingie into the dimmer receptacle.
3. Plug up to 3 cages into the 1-to-3 outlet thingie.
4. Repeat steps 1-3 for the other cages and other dimmer.
The dimmers will work out fine until you get the "smarter" temperature controller.
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Mark