You would actually only need another inch, plus a frog-hair just in case. It only needs to be 16" to be legal on a rifle, as long as the total length of the gun is 26" or more.
You could just sleeve it with a bloop-tube if you didn't care for a muzzle brake.
I would suggest either pressed on, or threaded, with a blind pin to prevent non-destructive removal. (Drilled on one side, pinned, and then the pin entry hole tack-welded shut.)
All out welding would not be good for the barrel because it would get it too hot.
Silver solder or soft solder would not pass muster if the ATF were to see it.
I haven't a clue who might be able to do it in your area of the country.
rcmodel