I just received a couple of reels of tape in Scotch 60s-era "Slimline" cases. The edge rubber foam seals were a complete mess, but those are easily removed. The center rubber "boot" however has kept its expanded shape, probably over decades of storage. So the result is that the only way to get the case open is to pry the "boot" out of the center.

Does anyone know a safe way to shrink rubber back to its original shape.

Here's the part I'm talking about: