My 18 came with no windshield, so I had nothing to work with.
Ordered one from these guys, as they have all the original Seacraft windshield specs.
http://www.updplastics.com/
Was kind of spendy shipped to the Cape from Florida (I think around $250?), but it fits perfectly, and was very nicely made.
I opted for the extra thick glass too which added to the bottom line.