Here is an Ammo box with a neoprene cover. You find these occasionally. This one sold off the site a couple weeks back.
You can see it’s stitched together and looks homemade. The neoprene is the same style you would fine on 7oz tank covers and vl-2000 / revolution loader covers.
The back portion had a tab of velcro and there was likely a top piece that somehow attached. I haven’t come across too many of these neoprene covers but there are all kinds of wild modifications that people would do to ammo boxes such as cut the sides open and put foam inside and then glue them, and more rational modifications such as add butler creek scope covers as flip up lids.
Find ammo boxes on baccipaintball at:
And find the product page for this cover that sold at: