heat pads for phals

[jdavis]

I live in a very coastal neighborhood of San Francisco (cool, fog, wind, salt) and I have 5 phals in my home. I want to use a heat pad for them because the cool temperatures (in my home as well) seem to be slowing their growth down. They are all in clay pots with lots of holes.
I do not want to use plastic pots, although that has been strongly recommended by the SFOS.
On the heat pad, do I put the pot directly on it or do I use feet or a saucer for the pot? What temperature is optimal for the setting on the thermostat? Do I keep it on all day and night?
Thanks. for any help.