Only start a few plants a year? A zippered, plastic storage bag — the kind bedspreads are sold in — makes a great temporary greenhouse.
Reinforce the bottom of the comforter bag and keep plastic flats of seedlings upright by putting them in a cardboard tissue or shoe box with the top cut off. Stick short bamboo stakes into the corners to hold the plastic upright, just like you see at left.
Then put the structure outside like any other greenhouse. Just open the zippered flap on sunny days, to keep too much heat from building up inside. After the seedlings are transplanted, wipe the clear plastic clean and fold and store it for next year.