12 month ago I decided to try neoprene as a lightseal in my Kiev back (old type) - I cut a strip of 0,5mm thick foam to size, installed it, inserted the darkslide and let it rest for a week. After that the foam adapted to the slide and sealed the slot tightly.
It worked very fine, better than the original Kiev/Hasselblad seals with foam and mylar foil - until last week. I lost a whole roll due to a light leak, the neoprene shrunk lengthwise and opened a tiny leak, just enough to fog the negative diagonal. :-(
I remember when I cut the stuff that I wanted to check the seal after 12 months... I didn´t, and I paid for it. <g>