Do you have electricity in your greenhouse? My DH wired a wall thermostat to a plug, and I just have the cheapest little heater with a fan, which kicks in when the temp goes below 40o. Works fine for the tropicals, geraniums, begonia bulbs etc. Gets a little crowded in there! Mine is also plastic 6 x 10 or so. The only problem I had was last year I got fungus gnats, in spite of hanging bug strips, but they went away when the weather warmed up.

No I don't have electricity in the greenhouse but it's very close to my outside outlet so I can get a heater with no problem. Thank you for the advice.

I do a similar thing to Novita. The fan runs 24/7 and I think that keeps the greenhouse from getting too mouldy. It always smells fresh throughout the winter.

You might look into radiant heat panels. DS is considering them for certain rooms in his horse barn.

Here is a link that might be useful: heat panels