Four individual sides, made to the thickness of the gunport frame, glued together and clamped. Then I cut sections of it, the length of which depends on the hull shape. The length is say 1.5 times the hull thickness if the hull is close to vertical at the gunport, but can be 2-3 times that thick when there is a strong tumblehome. The position of the tube section is near horizontal or close to parallel to the deck at the gunport.
Janos