The next question was 'how to display it like it's on SERENITY's hull', without getting bogged down with minutia.
A tuna can became a notional deck, with a foil yogurt lid inverted and glued to it's bottom.

A solid spray of khaki (as it appeared in the movie) with a flat black wash on the barrel and sights finished the finishing.

The cable tie-downs were a head-scratching plot hole, since I couldn't figure how they kept it on that fast moving spaceship but hey... it's Hollywood.
I found some thin braided wire in the shop, poked some holes in the bottom of the can and two of the too-big holes became cable-pass-throughs (so what you see tying it down actually holds the gun to the can).