Do you want to try using a waterbed heater? We still have the queen waterbed and two heaters. You could set your plants on top of the waterbed mattress...with the heaters below. The heaters will warm the water and heat up the roots real well 75-100F. I'm not really allowed trying this out per hubby. So if you're game, I can box it up for you and you can pay for postage.

Heating cables are widely available. I have bought them from Park Seed and Charlies Greenhouse, mail order. I also have bought the same thing at my local garden center. Al