Unfortunately, it didn't turn up today so I can't spend the evening fiddling with it!
Light tightness isn't something I had thought about until you mentioned it. I'm fairly certain my Minolta lens mod doesn't have any gaps but I will have a look.
I have access to some serious metal removing equipment... possible a bit too heavy duty for this. I have a friend with a military history museum who collects and rennovates military tanks. He has a lathe which is just about small enough.
I do have a smaller lathe myself but it has been ignored for too long in my shed and has seized up (a project for another day).
I will certainly give it a go. If my lens doesn't fit then I will find something else to go in it.