Old timer, what you do right now is what I do - I leave a heater running right now that keeps the boat between 50 and 60 degrees irregardless of what the temperature is outside.
Kevin E I need to clarify what I posted. The 75 degree limit is where I turn off the heater not what I keep the temperature at, the rest of the time the heater runs continuously. Because the heater is so small it is only able to maintain the temperature in the shop between 5 and 10 degrees above whatever the outside temperature is. As a practical matter you only need it between freezing and 75 degrees to keep the dew point from causing a problem ( I haven't bothered to install a low temerature cut off but with energy prices up I'm thinking of it).
To give you an idea of how well the 5 to 10 degree differential over outside works I had a leak in the roof last year (caused by some tree surgeons walking on the old asphalt shingles) that let significant water onto the shop floor (not anything else thank goodness). In spite of the extra water I still didn't have any rusting on any tools!
I don't know how big your current space heater is but it's probably much more than you need for what I'm talking about. You'll need to experiment, probably with several small closet heaters and a simple inside/outside thermometer to see how little continuous wattage you can get away with. Once you're done you'll save a lot on your electric bill and won't need to buy any fancy control equipment.
By the way we use three smaller heaters in closets in our house in the summer to eliminate mildew and they work perfectly.
P.S. I don't know how many or what kind of heaters you have now but if you have more than one there are two ways to reduce their current drain. One is to install a power diode in them. The other is to hook them in series. Or, do both. This could save you having to buy more stuff while cutting your electric bill significantly.