You might consider the use of patio doors, available for free, if you're lucky and look hard enough, in most metropolitan areas.
Most patio doors are made of dual-pane, insulated glass sheets, usually about 36x80 (a perfect fit for small trucks and vans) and are ideal for applications such as south-facing trombe walls and greenhouses. Their light transmission rates are excellent and they last 'forever'.
There are plans on the 'Net.
For a little extra inspiration, you might look at this...
I have about eight of them leaning against my studio presently, ready for the cold weather.