I use a heat pad designed for reptiles and it generally keeps the rooting chamber at 20 degrees higher than ambient room temperature. I keep the pots of cuttings on a ceramic floor tile that sits directly above the heater.
You could also use an aquarium heater inside a jar of water or if it's an aquarium you could get a submersible heater and simply keep a couple inches of water in the bottom of the aquarium.

I would think that any temperature between 80-85F would be appropriate. I am sure my rooting chamber got hotter than that though and the plants loved it.

Mike

Thanks Mike. I really appreciate the info. I've always rooted cuttings outside during the Summer where it was in the 90's with no problem. I took advantage of Joni's December sale. I've placed the cuttings in pots in seed starting domes but the night temps are kinda low right now. I think the reptile heating pad was a good idea but wasn't sure how hot it got.