[Aleshea]

You could also get a heating pad to put under one side if the cage and keep it on low.

 

[Aleshea]

my cages have a plastic bottom too and as long as you keep it on the lowest setting it wont warp the plastic but its just a personal opinion.

 

[Aleshea]

I just use a heating pad on low but make shure you only keep it on one side so they can get away from the heat if they need too.