Just thought I'd share what I did to utilize the door of my Stack On 8-gun cabinet. The problem with stack on is the locking mechanism is exposed, which makes it hard to utilize the door space. The organizers they sell use magnets that can't hold much weight.
The cabinets do have a hollowed out area on the door, so I took 1/2" plywood, cut just a little larger than the opening (to fit in the metal lip that's on the left, right, and top. I wrapped in felt and, with some wiggling, got it into place under the lips. If you cut about 1/8" taller and 1/4" wider, it should be small enough to fit in, but the extra will fit right under that lip and hold tight. I didn't need to screw into the door to secure. I took some metal L-shaped hooks, sprayed with several coats of liquid plastic and screwed them into the plywood. For mags, I just stapled some elastic strap and they fit right in.
My revolver's barrel was too long, so I had to hang that vertically, but everything else fits great.

Sorry - not sure why the photos are sideways or how to change it.

That looks good,i never thought of doing that,i guess I picked up another project to do

It didn't take long at all, other than waiting for the coats of the plastic to dry on the hooks. Time actually spent working on it was a few hours at the most.