The cheap way would be to get a carbon and fiberglass piece like the one above, and get it bonded in place with urethane adhesive. In other words, it will mount like a windshield, but you can use urethane adhesive that's quicker setting and not spec'd for glass mounting. I made one like that for Jay's 3000GT but it mounted as a functional replacement for the stock glass, not bolted down like the one in the picture. A metal one would be better, but would take a fair deal of metal fab if you want the shape to match the stock roof line. The nice thing about a welded-in metal piece is that it will help to reinforce the body, unlike a sunroof or a carbon piece. A stiffer chassis is always a good thing if you're going to autocross..