I have a hard time finding material locally, so I've been using onlinemetals.com.
They have brass flat bar in 1/4" x 1-1/2" for the uprights. I also found this in aluminum.
I only have rudimentary tools (no lathe or mill), so my plan is to use two 1/8" x 3/4" rounded edge flat bar (aluminum) and rivet the Lexan in place sandwiched between the strips for the bottom trim (with the rubber trim on the bottom).
It may sound funky, but when I looked at racing windshields I saw similar approaches.
My objective is to make a full-width 6" high windscreen that uses the standard upright mounts---as well as the center bracket---so I can interchange windshields without drilling extra holes in the body.
I got some heavy posterboard from the local art shop and made a full windshield template. I'll use this to cut and trim the Lexan (polycarbonate). Lexan in widths larger than 48" (you need ~53") are hard to find, but TAP Plastics offers wider sheets.
http://www.tapplastics.com/product/p..._sheets_ar/517
DD