The idea is – fast deployable and protected from elements and dirt, standalone shotgun ammo pouch.
First attempt is not nearly ideal, but indicates that it’s right way.
Operation is simple and natural, open the flap:
Take the pullout piece and, hmm, pull it out:
Tray is appearing with velcro on its back (flap is disappearing):
Rotate the tray, velcro stick onto base, cartridges are ready to load:
Sadly, prototype is too wide, it can easily lose few santimeters with no harm.