I've been using clear plastic sterlite storage containers for the last several years. Just get them tall enough to allow the seedlings to grow nice and tall. I use a brick to prop it open on warm days (if we ever have any). and put a few bricks on top for windy days (we have plenty of those). They last 3-5 years, then will one day shatter just like glass. Quite a mess to pick up, but it's worth it for these cheap frames. Get them at wal-mart, big lots or dollar stores for the cheapest ones.
I sometimes use those for wintersowing. I drill holes in the bottoms and about 1" up on the sides, then fill them with pots of soil and seeds, and leave them out to freeze and thaw during the winter and spring. They do seem to get brittle awfully quickly, though. I wish I could find something comparable that is made to withstand the UV rays! Can't you just see all my plants wearing Ray Bans? LOL
Thanks for reading and commenting! I love how everyone shares ideas and experiences here on DG. . .that's what makes it such an interesting community!