Sorry to disappoint you, but it's basically impossible with a struture that small to gain enough solar gain to keep the air temp warm enough to grow year round in your zone.

I'm in zone 5 and have a HFGH 10x12 and use an active solar heat sink and can barely keep it from freezing AFTER march 1st. I shut down my HG around Thanksgiving and start up again in march.

This response is a little late but hopefully this will help if you haven't found a way yet...

If you can use manure in the greenhouse maybe in a large container and put your plant boxes on top. The manure generates heat by decomposition. I do this outside for winter with a small grow box and it works all winter for me. But you would need to do something to generate heat not just maintain it like with the black tubes (which is another great idea but probably won't work by itself)

Tammy :-)

there are three ways:

Insulation, insulation and insulation!

Get from Home Depote rigid foam boards, using Velcro tapes, every day after the sun is down, put up the boards.

dcarch

6x8 is pretty small. You really need to be bigger.

University of Vermont has some good info on "high tunnel" greenhouses. An HT is low tech greenhouse.

University of Missouri has a good design for a barrel moderated greenhouse.

Barrels don't stratify as much as you'd think. During the part of the year you want heat, the light is striking them on the side. With the warmer water on the side, it powers the circulation.

If they are open top barrels, put a divider in them. Coreplast works well for this. It should fit the diameter of the barrel, and about 2" from the bottom and top. (Put 'ears' on the bottom to keep it from sliding down.) This
divides the barrel into a front and back half. Now water has to rise on the front and fall on the back. May have to play with that 2" dimension. (You want 2" of water over the top edge, and under the bottom edge.)