Has anyone tried 3M weatherstrip adhesive? It comes in black and yellow and seems as though it might be suitable for this purpose if thinned. I have never tried to thin it but I would expect that lacquer thinner or acetone would work. It's extremely flexible when dry and might be worth a look if that hasn't already been done. I believe that it will not further harden over time, at least based on how it acts when you try to remove weatherstripping that was attached using it.