For years I used a plywood over-bath arrangement for my trays, and a couple of coats of good exterior grade varnish was all it needed. I use an electric oil-filled radiator in my space with the thermostat set to 68-70 F. I don't run it all the time, but with a bit of planning it brings the temperature up. Admittedly we don't get down much below freezing here very often, but keeping the temperature no lower than the mid-40's protects the chemicals. If you have a device with a low setting you can use a plug-in timer to warm the space up in good time. Just watch the power rating of the device - many are designed for lamps, not heaters.