Refoaming cameras is cake! Its the cleaning that is annoying.
Best to measure all the pieces that you need before cleaning. Once you scrape and clean out all the gunk (I use naphtha to break down the foam and old adheasive), you can apply the foam seals. I use a home made trimmed bamboo skewer which doesn't mar (its also included in many precut kits).
I found a brand called foamies, which makes black 2mm thick adheasive backed closed cell foam (they also make other colors), which is really cheap, and comes in nice large pieces. I measure and precut the foam with a sharp razor and metal ruler. application is basically removing the backing, licking the adheasive (lets you position it easily), and working it in with my hands and another piece of clean bamboo skewer. take care around the film counter reset button. removing the back film door makes it easier too. for the mirror box, just use a razor and trim/scrape off the old foam, and apply foam right over, no need to clean. Also be careful with the amount of naphtha you use, you dont need a lot. dont touch things like the mirror or shutter curtains.
So far, I have done a Olympus RC, and Nikon EM, and a Nikon F3HP, and a Nikon AF1 gel filter holder, a Thomas Duplex safe light, and still have half a sheet of the foam left. I got 3 sheets from a seller on ebay for $5.50 shipped.
You can find them labeled as Darice Sticky Back Sheets, or Foamies - Foam Sheets Sticky Backs. find them online or in a craft store.
edit: meant to say I still have half of the 1st sheet I bought left!