I have the standard 1.25" film canister, but I need to make it slightly smaller, around 2-3mm smaller. Is there any way to do this? I need to keep the tube light-proof(no light leaks)
Making a container light-tight is much easier than trying to resize a container and lid no matter how slight the alteration.
The hack is rather easy after you find a container that 'fits' your requirements:
Block light by painting the inside and/or outside of an appropriately-sized container with opaque enamel (or other material) to suit.
Hack (off) the slack.
If the container you have is perfect except for being overlong, reduce the length of your container by cutting the required amount from the body of the container and fasten the two halves together. Removing extra length from the middle gives you the most room for a strong mending splice and preserves the lid configuration on the end.
There are very aggressive adhesive tapes that are opaque. Metal foil will increase the ability of any tape to resist light leaks. Additionally, you might decide to use epoxy for more secure (and mechanically not so flexible) bond.
Try to find one of the old style screw-cap metal film cans. These were the same inside dimension, but the metal was a fraction of the thickness of modern plastic film cannisters, leading to a smaller outside diameter. Since they're made of solid metal, deep drawn more or less like a modern beverage can, they're 100% opaque, weigh little if any more than a plastic can (because the metal need be only thick enough for handling) -- and most had a rubber seal ring in the cap, making them airtight (at least when new).
These are no longer available new, but they pop up from time to time on eBay as collector items. They should still cost less than a roll of new film.
I did something similar on a larger scale - I had two opaque black bottles from industrial peroxide, and after cutting the tops off wanted one to nest inside the other to make a bike tool case (they're the same diameter as a bike bottle).
I shrank the inner and enlarged the outer. Shrinking was slightly easier. Of course I used heat, but the key is to apply even pressure all round. Use gloves/tongs/piers for handling hot stuff, especially as there's hot metal in this method.
For this a Jubilee clip (hose clamp) works well. Wrap it round and do it up tight, but not so tight the plastic buckles. Heat the container, e.g. with a heat gun. An oven can work too. While still hot, tighten the clamp some more. You might need to repeat a few times.
To enlarge the outer, once the inner had fully cooled, I heated the outer, pressed the very edge outwards a little with a length of dowel, and then forced it the outer over the the inner. This wouldn't work for a film can cap.
Shrinking the base would be harder, but should still be possible using this approach, though it would probably end up bulging.