So we decided that we would build one this fall. We contemplated making it a 4th use on our pumphouse/chicken coop/pole barn structure we're constructing (ok... so we're not constructing yet but we have it designed and some materials purchased), but decided it would be too risky to have goats so close to tender (and yummy!) young plants.
Then we decided to look at http://www.farmtek.com/ for some greenhouse ideas. They have a product that we just can't beat with a DIY.
This baby comes with 2 covers, one for winter (clear PVC for maximum light transmission) and one for summer (60% shade cloth). It has 4 shelves giving 14.25 square feet of grow space. All for less than $60 (shipping to here is about $15)
With stacked shelves, we'd be very dependent on morning and afternoon sun as only the top shelf would get any sun at noon, but I still think it would be worth the money. And were we to DIY it, I'd probably spend $60 on fabric or visqueen and a bunch of time ending up with a product with lots of air leaks.
So we have not yet ordered this next homestead upgrade, but if anyone local (Volusia County) is interested in the same or similar farmtek item, there's a bit of a price break with larger orders as well as some savings on shipping. So if you're interested in something, let me know. Or if you know a way to do something similar for less, especially if using recycled materials, please share the wealth.