First off, welcome to the forum. I know nothing about cannons so others here will chime in with good info. But I think that is a good something-or-other; please don't "trash it". Someone here will put it to good use.
Much easier to have a flat but angled cover. Then fasten a thick piece of brass with screws, pins and/or glue. I keep a small stash of 1/8” bar stock. Once fastened and secure, just file to match the angle and profile of the butt plate.
That is a beautiful chunk of maple. I think the color and contrast is perfect.
May I suggest a couple of more coats of the Permalyn applied with some white (extra fine) Scotchbrite? Then once dry and happy with color, add a rubbing of paste wax.
Well done Koauke.
Just continue sanding with finer grits, 220 thru 320, then stain of choice.
I do not recommend steel wool until after nitrate treatment, unless you want little rust spots or freckles. Steel wool is OK after aquafortis. Or just switch to proper grits of Scotchbrite.
Thank you SDS for that nice compliment. But I will still strive to improve, always reaching forward but probably never attaining the abilities of the likes of Lucky or PathfinderNC or others.
Rick
As an alternate, use a screwdriver and punch a couple of hole to pour from, leaving the lid in place. This approach minimizes open top surface area. But ultimately, just poke, punch and discard.