MO,
The "Traditional Muzzleloaders" know about these.
Picture a full moon clip for a .45ACP revolver. Not your forte, eh? My grandkids have these cap pistols that use a plastic ring of percussion caps . Have you seen those?
Cut a circle of hard leather, about the size of a silver dollar, punch holes (of a size that will firmly hold a cap) around the edge, without breaking through. Slit from the holes to the outside, and cut away material between holes that interfere with reaching the nipples. The thickness of the leather and its hardness are critical to it being managable while doing the job.
Clear? As mud? Well, it covers the ground! :wink: