I have a lathe and a mill in the garage. Just a hobby though. I'd be happy to help.
What sort of cap are you looking for and what size is the outlet of the beer keg?
Given the rolled top of the outlet shown in the pics something that fits inside the outlet would make more sense to me. Would a big cork or bung not work?
HIKING CAMP STOVES, COOKSETS, CARTRIDGES - StainlessSteel Cup MH150(0.4L)
£3.49 - HIKING CAMP STOVES, COOKSETS, CARTRIDGES - StainlessSteel Cup MH150(0.4L) - QUECHUAwww.decathlon.co.uk
Doesn't a cup like this would work?
Such stuff exists in nearly all sizes.
When they filled some with sand on the fire station they made wooden bungs and hammered them in still surviving after 5 years
I cannot remember when you unscrew and take out the valve part, you may be able to remove the pipes etc and seal the openings and reuse cap. The lads on station went for the easiest option