Always found the base of a shotgun shell a handy way to cart them around.
I was thinking the same thing- in a shotgun cartridge where they belong.
Now, if you asked me how I carry extra flints or #11 caps- loose in my shooting bag or in a capper, respectively. Musket caps, for when I unlimber a percussion musket, carry well in either an original or repro Civil War cap box on the belt. Would probably work well for 209's too.
Shooting bag= that which contains everything necessary to load/maintain the rifle in the field.
Possibles bag= a separate bag into which goes the rest of the junk one might "possibly" need for a day afield.