As part of restoring 581, I had the original window rubber re-popped. This fits at the top, bottom and rear edge of the removable windows. I have surplus due to the minimum run required, so if anyone is interested, an uncut length of more than enough to do one car is just US$80 plus shipping.
Also looking at making the plastic extrusion that was used for the fixed position vent or quarterlight window.
Cheers!