Here's an under-tank heating pad from petco. Details say it can raise the temp 5 to 10 degrees. One review mentioned using it for a 40 gallon tank. Not sure if it was a breeder-size or not but it may help...
I just joined and this is the first post I read. I know it's been a few months since she laid and was wondering how it all turned out... I'm literally on the edge of my seat. Hope all is well with her. If they did hatch, I think you'd be a fantastic grandma to the babies!