Our hoop house is 15' x 40' covered in plastic. There is no external heat source - just mother sun!  Barrels of water, stone and brick are used to hold heat through the night.  The thermal mass can hold off a freeze in the spring and fall.

Worm bin under construction.  Walls are made from standard thin cement blocks.  and the whole thing is set into the ground so it does not take up any extra greenhouse space.

Placed a simple wood lid on top reinforced with 2x4s so that it can be walked over.