They are made from LEXAN which is a trade name for polycarbonate. Buy 3/16 or ¼ inch thick clear. Usually you can get a scrap piece for around $10.00. Leave the protective paper on until you are completely finished and you won't scratch them. Trace the pattern and cut the Lexan with a band saw, jig saw or hand held coping saw. The stuff is incredibly strong and almost unbreakable. The use it for bullet proof applications. Sand by hand or with a belt sander or use a file to smooth up and round up shape. If you want you can then buff edges with tripoli or similar buffing compound . This will produce a real professional looking edge. Depending on the type and location of your turn signals you might have to alter shape as shown on the pattern with the extra curved line.
I used u-bolts I bought from MSC ) Manhattan Supply CO) Their part number is 55907848179. They are 5/16 x 3 x 3 11/16. I also bought some 5/16 in I.D.tubing in black and slid it over the bolts I then used 5/16 stainless steel acorn cap nuts and washers.
When you attach the lowers to the bike set them at an angle similar to the curve of your windshield and tighten them up. Do not be afraid to tighten them . The plastic will bend but not break I also used blue Loctite thread locker on the nuts so they would not vibrate off.
NOTE: I did this mod as well. I purchased 1/4 in. polycarbonate at a local glass store. That was the only place I could find the thicker size. Home Depot has the .093in size which is too thin. I also bought stainless steel u-bolts and cap nuts at Home Depot. Since I have chrome trim, I used clear tygon, plastic tubing. These ended up looking pretty good.
WARNING: Don't overtighten the ubolts. You will warp your fork cover and it will rub on the fork outer tube. You really don't need these to be really tight for them to stay stationary.