Since I don't have a greenhouse yet I make great use of my Cold Frame that was built 10 years ago. I use it to start seedlings a few weeks early, harden off all of my seedlings and store greenhouse purchases until weather permits planting.
It was quite pretty in it's youth. The lids were picture framed with mortise and tenon corners. The panels, salvaged from a friend's commercial greenhouse, were set into grooves. All these details made the lids attractive, but also created some problems. The panels were recessed below the wooden surface which made a great place for ice to collect in the winter. The weight of snow could bend the panels enough for them to pop out of their grooves. Water accumulated along the lower edge and any moisture or dirt that got into the channels of the panels, from either end, was trapped there forever.
Greenhousemegastore.com sells reasonably priced 2 foot by 4 foot twinwalled polycarbonte panels in packages of 5 with free shipping. And they also offer a 10% discount to listeners of the Joe Gardener podcasts. These panels are perfect for a do it yourself cold frame project. They come in either 6mm or 8mm and I ordered the 8mm. My husband was really excited to hear that their size (while convenient to ship) meant the lid would have to be built in three pieces instead of two - but not excited in a good way.