The back of the fairing is one piece of coraplast about eight feet long. If you have a heat gun you can bend it into a nice curve and have a very nicely formed piece of coraplast. Oh well, no heat gun, here's my solution.
The support for the back piece of the fairing is one piece of PVC tubing with a Tee on each end. This is placed somewhere behind the seat of the recumbent. On my Sonic I just fastened the PVC pipe to my rack with two hose clamps. The piece of coraplast I used for the back was 26 inches by eight feet.
For the back piece of coraplast I measured the width of my rear rack. I then made two parallel cuts halfway through the coraplast in the middle of the sheet. These cuts were through just the outer skin leaving the inner side intact. I measured from the back corner of my rack to the Tee. I made another cut on the outside straight up and down. I repeated this procedure with the other side. I positioned the side and held it in-place with duct tape. I again used my level to make sure it was parallel to the ground. I then fastened the side to the Tee with three steel pop rivets.
It doesn't formed a nice with curve but I have a beautiful surface to mount the reflectors on.
I then removed part of the rear sections of the fairing to reduce its side area. Since the back was formed with angles instead of curves. I just removed triangular shaped pieces from the back.