I live in Nebraska so im dealing with the same temps.
Along with the flexwatt i use a ceramic heater on top of the cages.
I keep all my pets in the basement where it cooler but i have an electric oil filled radiant heater and it keeps the room at 79 degrees.
For more backup i have some heat rope from big apple and i can spool it up and cover with aluminum tape to keep it spaced evenly.
The snakes love the ceramic heaters though. They sit under them all the time.