I use 3M 850 tape for all my bellows. It's very strong, blocks light, and really flexible, the adhesive stands up to cold and heat without changes. Very thin. The bellows on my Wista 45 has been patched with this stuff for probably five years now and it's still perfect and it's on at least three other bellows.
It's pretty expensive, but worth it.
My favorite feature is having an old washer without circuit boards that cost more than a new washer.
It's not as fancy, or as convenient, but it's a trade I'd make again any day.