I don't have a suggestion for this year, but what has been working good for us is a rocket mass heater.
I installed one in our hoop house a couple of years ago and now we start our seeds in there the first of April. We live in zone 3 so
not too much goes into the garden before the 1st of June. Even then we have lost some gardens because of a late frost in June. The heater mass is about 28 feet long and 2 feet wide. I put about a 2 foot tall shelf on top of that to drape some plastic over. At night we put some old sleeping bags over the top to retain the heat. After the initial warm up I'll usually fire it up for 1 1/2 hours each night.
This systm has replaced many lights and shelves that used to be in our garage.