Michael McKelvey posted:@IaM-Ray, here is the picture of the zippered rear window. The top is in its flaccid state for the winter. Visibility out the rear is a bit improved with the change.
Nicely done Michael... Ray
I do not drive in the Winter as I am buried in snow. However it is quite cold in the Spring and Fall. I modified the curtains to fully enclose the car, I have convex mirrors, and I added the center bow lock down on the windshield. I drive with the top up in any cold weather. My car does not suffer any buffeting and curtain blow through. Also in a driving rain it does not lead through the windshield. And it sort of gets warm until the temp drops below 36F.
Many of the original 356 Cabriolets had zippered rear windows and a LOT of IM roadsters do as well. Original 356 Speedsters only had zippered windows if the owner was familiar with the concept from other 356's and knew of the advantages (just like us).
I've ridden in several "zippered" cars including both originals and also John Estes' IM and with the rear window down the top billows up and stays there (as it should), does not buffet and the airflow through the car is wonderful - no internal buffeting that I could discern and the top stays nicely taut. Air comes in from the side and right out the back - a nice, cooling effect. It makes hot day cruising way more enjoyable AND keeps the sun off your neck, arms and legs so you don't have to wear a long-sleeved shirt or long pants to cover your lower thighs and knees.
Beck speedsters, with their articulated top frame, may keep things tight enough that the top doesn't buffet at speed, but unless you can lower the rear window, the airflow through the cockpit just isn't the same.
@IamRay
"Bob, adding the zipper to your build was a great decision on your part, most times I put a microfibre towel to keep the plastic window from getting damaged, have you tried that. I'm just slightly OCD, and XYZ like Marty. 
"
Yes, Ray, I do use a towel on both sides of the folded down plastic.
I figured that out all by myself, too. 
AGE is useful after all