@Coen: Great work! Thanks.
If I read your drawing correctly, it looks like you've used a classic assembly where the oring requires compression to work. I do a lot of technical diving and underwater photography and we generally avoid these types of orings if we can. There are two main reasons for that:
If the locking mechanism is not screwed tight, or if it becomes slightly unscrewed, the compression is lost and the the thing starts to leak. In addition, plastic threads are quite fragile and can easily get damaged by repetitive screwing/unscrewing or if you think that a leak will appear if you don't screw it really tight;
The oring can sometime "jump" out of its seat during assembly. If that happens, it's something that's very difficult to notice;
Instead, we prefer orings that sit in a groove located just above the thread. It requires a touch of lubricant (generally silicone gel), but it's much safer. I'm not sure if I'm making any sense and I've enclosed below a couple of examples. The locking mechanism here is a bayonet, but the oring principle remains the same:
Wide Angle Port - The oring is in blue on the far left of the picture
Adapter - The oring is the light grey bit above the bayonet locking mechanism
I hope that I haven't misread your drawing completely and that the above helps.