Back in the day the rifleman would have made his own cover using hide which was the correct thickness to form the cover so it would allow the frizzen to close securely to keep the charge in the pan in place. How thick is the face leather on thatIf you're looking at shooting at a range or other practice area then it's not really something you ever need. Or if you're looking at hunting with it then once you have a charge in the pan and the frizzen is closed wrap some thin oiled leather or some other water resistant but tight wr cover?
Even without a flash charge in the pan dropping the flint against the frizzen a spark COULD find some "dust" around the edge of the flash hole and set off th I'm not a hunter myself but I believe that legally a flint lock is disabled and made safe for transport even with a charge in the chamber by removing the flint.