We experienced the same problem with our '42 and believe that the aftermarket rubber gaskets should be glued to the cowl vent itself. After placing the gasket on the cowl, I put the weatherstrip adhesive on what you referred to as the bottom of the "L" and then gently closed the vent, until the cement set. We tried at least a couple venders and the gasket looked the same so this must be the way it is designed. The relationships between the gasket and vent surfaces are very different than an AD cowl vent.