Has anyone taken a piece of stiff cardboard the shape of the stock windshield, but taller/wider and laid it over the front of the shield, taping or otherwise clamping it in place? Bring the bike up to moderate speed, raise and lower the modded shield, and checked for air pocket and buffeting. Cut down as needed. Sort of "try windshield" like the shotgun trap shooters use a "try gun" to give the gunsmith a set of dimensions that fit them just right?
If the only option is to buy $150+ windshields that may or may not fit them, and many go through one or two before they find one that fits, there must be a better way!