The guy who will inslall the window glass in my studio doesn't know EPDM rubbert or closed cell used in Knightfly sketchs. But In the SAE site window construction uses "easy to found" material: Silicon and Cork or Rubber:
I think the problem with mounting the glass ON the neoprene is that it won't take the weight for very long. You might check into places that replace auto windshields to see if they will sell you the rubber. I'm pretty sure it's the same stuff.