Cant help with the gun mount but I do have a cargo box in the bed. Bought box at Wallyworld for 40 bucks and it does seal up pretty good. I added a couple of simple S hooks at each of 4 corners and use a simple strap on each end hooked to the loops inside the bed. I have a shovel, axe, rain gear, tools, extra oil and coolant, tow cables, all at about 45 lbs. Box has never come loose even during a rollover. Nothing has ever gotten wet, box has molded loops for a lock if needed.