I would think sections of outer plank instead of one long piece where the guns are as they will get cut anyhow for the
framing of the covers. Another idea is a bit of line chaulk on each barrels mouth, place wood against and lined up then
drill out the little blue circles you just made. Id drill them slightly larger to allow for moving the lathe in to its proper
possition. Fine adjustments may be needed.
Just my stain induced rambling... Yes, I been sniffin stain again on the crows nest tossing bananna peals down at unsuspecting
crate laden sailors