Mike Summit and Jegs offer a ready to go kit...or used too. I always bought a roll from a local plastic supply store. You can find the 8, 10", or 12" clear vinyl on the roll at any shipping supply company on line but the shipping costs so much it's worth it to buy locally if you can. THE STUFF IS FRICKING HEAVY. It takes about 10 or 11 10" strips to close of your rear 8ft wide sled door. I did a man door and a rear and it was nice to be able to keep them open but also maintain your interior heat. We just got an aluminum 3" x 8" flat stock and bolted the plastic sheets to the flat stock and then hung it in the trailer. ONCE AGAIN...CAUTION AS THE NET WEIGHT OF THIS STUFF IS SUPER HEAVY AND YOU NEED EXCELLENT MOUNTING SUPPORT ON THAT AREA OF YOUR TRAILER (mine fell down twice....and ruined the cheapo factory spring cover) AS THINGS GET VERY BOUNCY BACK THERE.
Now all of that said there is a far better way. Shawn Atkinson RMSHA racer out of East Idaho or UT makes a part canvas and part clear vinyl rear door cover. MUCHO BETTER and once you spent your time and money building your own this is almost as cheap...way better quality...lighter..and well just BETTER!