I had a small winter garden a few years back. I used a popup greehhouse (commercially available) that folds for storage and when used was about the size of a pup tent. The sides zipped open for access.
For light freezes (down to maybe 28-30 degrees) no heat was necessary. Just zip it shut. I had to unzip during the day as it could get pretty warm. For hard freezes (25 degrees for more than a couple hours) I used an incandescent bulb in a drop lamp. Consider a 60 watt incandescent bulb is basically a 55 watt heater. It was more than enough. I would suggest going with 40 watt or lower if you can find them.
I can't wait to see pictures!