Get a UV resistant plastic. This is more a hoophouse than greenhouse right? The difference being a hoophouse is usually not heated. You can do what this guy did and dig a few feet for more stable temps: http://www.bananas.org/f2/my-new-banana-house-22336.html

LOL...that would be awesome to dig down like that...but I am done building. Just a few more touches here and there before I put on the plastic. I already have the plastic, so I am going to go with the regular 6-mil for now. I will make the investment in real GH plastic in the future. Right now I am on a mission to see id I can actually finish this with "just materials I have on hand."

Here is what I did today.

I have a 2'-6"" x 12' work table with tons of space for storage underneath. That leaves a bed 2'-6" x 9' bed on the end of the table and a 2'-6" x 21' bed on opposite wall. The back end connects the dots for another another bed area of 2'-6" x 3'.

Now before I put the plastic on, I want to infill the beds with soil. I have 2+ year old composted horse manure and garden soil. My plan was to mix the two 50/50. I had planned on laying down cardboard in the bottom of the beds, layer a bunch of leaves and then fill up with the 50/50 combination. My garden soil is pretty heavy, but I have worked compost into it for the last 3 - 4 years...so the clay is starting to give over to goodness. I might add masonry sand. That combination would be 50% compost, 35% garden soil and 15% sand...????

Any help or suggestions are welcome.

You might want to insulate around the outside of the kneewall, piling up dirt or strawbales around it. Since they are raised beds your soil mix should be fine. Keep us posted on how it goes. I tried using painters plastic out here it was friend in one season (we have 300 sunny days a year though)