OK, I have two sorts of film canister here. The black one with a grey lid, has a "lip" on the outside which retains the lid. Not marked, but I would think this might be enough of a retaining flange (only needed on the inward aspect) to hold it in place; and it is nice and smooth on the inside.
The other is the translucent "APS" film canister which has an inside- fitting lid and no outer flange. It is a bit flattish anyway, smaller (fits inside the other) and the shape might be a problem.
In either case, you would cut the bottom off the canister. As well as making it possible to pee through, it resolves a problem with air trapping inside.
If nothing else, you can probably hold it in place with adhesive tape? (Surgical "Micropore®" or equivalent.)
I understand you to say that your wife (as well as yourself) perceives matters to be "almost normal" with a non-retractile foreskin. It seems to me, that a non-retractile foreskin is not so far different from a circumcised penis - lack of proper foreskin sliding, less sensitive. As long as it's not sore
, I daresay it's pretty convenient for manual play (solo or mutual).
I can't make much of your fraenulum!
I daresay it was just stretched and probably torn - to oblivion. I don't think it is a terribly
important structure, but my main objection to people wanting to "get it cut", is that they should
be able to sort it out with patience themselves - if yours went AWOL during this process, I'm more than happy to leave it to you to observe whether this makes a perceptible difference.