Here are a few forum posts regarding aftermarket dew strips that can be used to replace the factory rubber portion of the door weatherstrip. You will need good molding pieces to attach this rubber to, but it will seal against the window when the window is up.
https://www.beretta.net/forum/viewtopic.php?p=134145#p134145
https://www.beretta.net/forum/viewtopic.php?f=27&t=23516&start=30#p137073
Restoration Specialties PN MR87X 96. Belt was $32 at time of install in 2014