Bob Drake has a 6 page illustrated set of instructions: Installation Instructions 01A-426-HK is the document number. I used it for my '41 cowl vent and everything works very well.
I went through the same issue with my 48. Came to the conclusion I should have fitted the vent prior to installing the rubber gasket.
It seems the gasket was lifting it. Wondered if the repo gaskets were thicker.