Better than either tempered glass or plexiglass, would be the same twin walled polycarbonate used in greenhouses. For a cold frame, 4 ml would be enough, although 6 ml would be more insulating. If you can't find it locally, check out:

http://www.farmtek.com/farm/supplies/prod1;ft1_building_material-ft1_corrugated_sheets_panels;pg104618.html

Or:

http://www.charleysgreenhouse.com/index.cfm?page=_productdetails&productid=1274

Twin walled polycarbonate panels are not particularly expensive, but if shipped in large panels, the cost of shipping could exceed that of the materials, so do look for a local supplier. If there aren't any in your area, ask any online seller if they will cut it to the size you need. If you can use panels cut to a size that would not dim out, it would ship UPS Ground at a faily reasonable rate, because twin walled polycarb is pretty lightweight and it's not the weight that drives up the shipping cost, but the dims.

The two seller references above are simply what first came to my mind. There are many others, perhaps at better prices, but now that you know what you are looking for- Google.

Try Lowe's and Home Depot, they both carry double wall polcarbonate in 4X8 sheets. At lease in Ocala FL. I think that is because people use to cover windows in hurrican season.

funnylady: You may be right. I never thought of looking for polycarb panels in my IL home centers, but then, we don't have a hurricane season. Neither does MA, but I did see some at a large farm supply dealer not too far from us, in Kankakee, IL.