Just looking, DuraCraft looks like it also has gun boxes that can be bought as accessories, I'm sure there are more manufacturers that offer them beside War Eagle and DuraCraft.
Probably the best bet would be to do some online searching and find a dealer that can order one for you.
I just added a few more things to mine, that gun/storage box will hold a lot of stuff!
3 life vests, a throwable cushion, 2 small ammo boxes with a bunch of things in them, 2 pin posts, measuring board, pee cup, stringer, lure retriever, large wrench for the impeller, piece of string trimmer line, fire extinguisher, fishing reg. book, and launch rope are all in there with a little room left to throw 4 rod holders in if needed. Without that box that stuff would be all over the place like you are trying to get away from.
This particular boat also has a smaller dry box on the opposite side, which I'm trying not to use except for things not needed for the boat itself and that will be removed on a regular basis, such as rain gear, extra clothing, etc.