For approx. $295.00 you can put together a solar only "hoop house"
(that's a google term for you) including some nice material from
Northern Greenhouse Sales to cover the hoops. Mine (12x18) uses 20
foot rebar covered with 1/2 inch pvc, and I built 2x4 endwalls and use
1x3 furring strips to secure the skin to (these are horizontal
battens). You'll get a good idea from any pics you see on the web
hoop houses.

You'll get about 5-6 weeks on both ends of the season with this, no
thermal ballast required (free-standing version). For a detailed
treatise on how this all works in a production setting, see Eliot
Colemans books..