2 wooden blocks
One straight, made from pine (soft)
one slightly tapered,,made from oak(?) so it hard
The (usually) tapered trigger guards , wider at one end than the other,,fit betw the blocks with the taper of the one
taking up the difference in width.
Any small inconsistancy is made up as the hard oak block presses the guard into the soft pine block and holds it securely.
If the guard is symetrical, non tapered, ect,,just use two straight blocks.
I have a bunch of different wooden vise blocks for holding parts scattered around the bench. One or more usually fits the part.
This is Purdey TG,,but you get the idea.