Mary B those look good but won't fit inside the enclosure they need to.
Joestue, i looked at a few ptc resistors, but couldn't find any with low temp curves. I didn't look terribly hard. I would be very interested if you know of some that start to cut out at freezing.
I ordered some bimetal disc thermostats with 0c rating and wire wound ceramic resistors 5w 330 ohm. That should give me 2w or so at 24v.
I'm thinking wrap the controller in insulation and 2w will keep it above -15c. I don't really like the bimetal disc because in my experience, they aren't good for many cycles, but I'll give it a try. It's cheap and available, and should fit in the space no problem.