I used florists wire for the gun barrels and the shields were cut from balsa. To make the ensemble suitable robust I mounted the guns in pairs on card. The size or the (rather over scale) emplacements can be gauged from the 1cm grid on my cutting mat in the photos.
For the Harbour Raid game I needed two single guns, so I simply cut down a pair I'd already made and mounted them on coins small enough to fit neatly on my Triang Minic breakwater ends. They're not terribly accurate models but in this case function definitely beats form!