Maybe you could consider a vivarium heating pad put under your pots ( with a layer of pea gravel between pot and pad. They easily hook up to thermostats and give a good temp control at the root zone ??
www.ebay.com.au/b/Reptile-Heat-Mats/1285/bn_27325081
www.ebay.com.au/b/Reptile-Heat-Mats/1285/bn_27325081