I travel with film all the time and, like you, am limited not so much by size, but by weight, for carry-on. In my carry-on I usually carry my Rolleiflex, Nikon FE + 2 primes, and film needed for the trip, which is often 50-100 rolls, depending on how long I am travelling for. If I am only allowed one bag then my phone, iPad mini, noise-cancelling headphones, any and all batteries, and important documents/wallet also have to go in there too. I am almost always at 7-10kg with this basic set-up. I also now travel with a photographer's vest in case I need to off-load some gear (especially film, which is surprisingly heavy when you have a lot of it!), since many airlines are so strict with carry-on luggage weight. Since I can't fit anything else in my carry-on, I always have to travel with a checked bag, which will have my tripod, all other photo accessories (cable release, filters, etc), my Holga and/or pinhole camera, and very rarely a flash (without batteries). No batteries whatsoever in checked luggage, it doesn't matter what kind.