The DOT installed, the material is tight against the window frame, wraps around the post and snaps to its anchor. This as weather-tight as an MGB is going to get.
True Fact! An MGB is a Roadster, not a convertible. Convertibles, at least in British parlance, had more complicated tops, usually with cant-rails and actual rubber seals against the glass. Think BJ7/BJ8 Austin-Healeys and E-type Drop Head Coupes.