Hi,
I assume you are doing this to meet some historical objective turning your fowler into a musket as was done quite a bit in New England early in the Rev War. A British carbine or French musket bayonet might be the best choice. You may have to buy a blank from the Rifle Shoppe for the best options. You should cut back the stock the length of the socket (often 4") plus another 3/8-1/2". The extra length allows 1/8-1/4" inch of barrel to protrude through the end of the socket. That helps create a snug fit.
Hopefully, you don't have to move barrel lugs or pipes.
You probably will have to move the front sight back and get rid of any barrel flare at the muzzle. You can substitute the bayonet lug as your front sight if you choose such that the bayonet blade is on the side as on British muskets. Alternatively, you can have the blade underneath so the lug is on the side and the front sight separate. Bayonets were hand fitted to the individual guns, which is why they usually had production marks that matched the gun to which they were fitted. I fit a lot of bayonets on reenactors' guns and I fit them as they were meant to be installed. They are snug, don't rattle, and should stay on the musket when you pull it out of the body of your enemy. To remove my fitted bayonets, you have to smack the bend in the blade with your palm to release it from the barrel. I fit the socket on the barrel and if it is a little loose, I heat the end of the socket red hot and peen it over with a ball peen hammer. I the then file the opening until it snugly fits the barrel. In addition, I taper the end of the "Z" slot such that the barrel lug tightens into the slot when it is fully installed. When you install the barrel lug for the bayonet, I urge you to make it with a slight dovetailed base and fit it to the barrel in a very shallow dovetailed slot. Then solder it in place with low temp silver bearing solder. Most of the originals were brazed.
dave