you can get a scabbard at a used tack shop for 20 bucks.

Back when I was a good ol boy scout, one of the other troops in our district had a BP cannon they had made from a scuba tank. They cut out the bottom, welded pegs in the sides (for swivel points on a sturdy wooden base), and had a fuze in the top where the oxygen spout normally screwed on. As a cannon of course, the top was now the back end, and the bottom was the front of the bore.

An 8.5 lb bowling ball fit just perfectly, wrapped in a cloth or rag, on top of a fourth of a cup of black powder.

And I tell ya, when they touch that thing off, you could feel it in the ground two hundred feet away. And that ball would go whistling up in the air about a thousand feet, and come back down on the side of the mountain. They really whistle like the bombs in cartoons. Then the scouts would scramble to the hillside to retrieve it and if it wasn't cracked or busted, they'd have another go with it. Didn't look very safe to me, but I saw them use it many many times and nothing ever went wrong.