I have not been able to find dimensions/template for the 30/31 standard windwing glass.
You might try calling Snyders to see if they could help with that.
It appears their part# is A-18252-B for the standard roadster.
Or you could order the glass and make the acrylic from it if that's what you want. Then you would have both. Link below:
http://www.snydersantiqueauto.com/se...-61951&x=0&y=0
FYI: I've never had the glass slip using original (concave) clamps and chasing the screw threads for good fit.