Has anyone ever used Coroplast (corrugated plastic sheeting–like that used in yard signs) to make, in part or whole, a small greenhouse?

If I remember correctly, this stuff is fairly translucent in spite of the make up of it. No, it's not clear, but for the bottom part of the walls, would be a great insulator and the ribbed stuff could be used on the upper walls and maybe even part of the roof. It's not in front of me and it's been a while since I actually had this stuff in my hands. Have a light meter? If so, check and see what the values/temps are. I do think you would need the clear stuff for at least part of the roof though, but the walls? No, I believe what you have is going to be fine. Think of all the people who actually use the 'clear' plastic to make temporary greenhouses. That stuff isn't clear, it does let light through, but it's absolutely not clear.

If it were me? You betcha I would use it! I don't know your latitude, so have no idea about your growing season. I do know that the stuff you have would be great for the upper part of the roof for blocking some of the mid day sun during the summer months. Even here in Maine, during the summer, a shade cloth is needed during the hottest part of the summer. You wouldn't if you use that stuff. Depending on the size of the building and roof, as well as pitch, you could do the upper 1/4 - 1/3 of the roof, the rest clear. During the winter, this won't be a problem as the sun is lower and will hit the clear part more. Just my thoughts on it. I got lucky with mine, I had a friend who gave me all of their old storm windows which I built the bulk of mine from. 2 x 3 framing and windows added to form the walls. Headers and footers, same as for a regular building. 16 on center for the rafters. good luck, I think you have hit the jackpot.