If we ever meet in the flesh, remind me to tell you about how the captains of Japanese industry sent a team to the UK to discover how such a talented race could make such a hash of achieving commercial success.
It is a true and fascinating story, but their judgement was straight out of “O what a lovely war” Lions led by donkeys.
I think I would at least research the possibility of a seal arrangement to suit the tube you have, even if you use a different design.
I needed a special seal once, so I made a metal mould in two halves and had them made. You would need to find a small rubber moulding company who would tell you what they wanted. As regards gun drilling, firms like Mollart used to do this and I am sure that a forage through the internet would find someone, but as regards getting the bore to a surface finish suitable for seals, they might not get it right first time.
Sorry I do not have a ready made answer, I have never been asked to do that job. Kind Regards