Kudos on the casework- as an amateur woodworker, I appreciate the art and the artisan. That case is functional art.
To your question: what I can envision might not be affordable or practical, or even possible- but you asked for ideas: a brass rod that is threaded for adjusting the length specific to the barrel length of the firearm; the breech end has spring loaded dogs that open into the rim recess of the chamber. The muzzle end twists to retract the dogs, and is secured by a cylinder lock.
Again, I commend your efforts to display the work of artisans past, constrained by the exigencies of the present.
Mike
