You may not need adhesive foam. I use sticky and plain and once you get either in the channel properly, they aren't going anywhere once the back has been closed a few times. Same with string or wool, closing the back compresses it. Just make sure it isn't so thick as to strain the hinge, or hairy to mess with the shutter. Camera seals are not an exact science, hence the use of a lousy material like foam to make a light tight seal. Mirror foam is slightly different, I would use an adhesive version first time round but double sided tape will do the same job.

If you apply the wrong piece of foam in a channel, just butt another section up against it and cut to length. I replaced a few camera seals with the remains of other kits, short lengths strung together like sausages and they're completely tight.