Without seeing the plans I won't comment on their authenticity, however there are several longrifle plans available in which the designer has taken significant artistic liberties. This pronounced flare may be just that. Some builders add flare to the muzzle end of the stock when building a straight barreled gun to give it the illusion of a swamped barrel. Look in RCA at some original Haines and other Lancaster guns and you'll find what an appropriate muzzle cap should look like - the flare should be parallel to the barrel itself. Now, if we're talking early German jaegers, they often had pronounced flared nose caps.
If you still want to make it flared, I'd shape a block of hard maple the same as your muzzle and bend the brass around that. You'll have to trim away excess wood at the muzzle to fit the narrow end of the muzzlecap in place.
-Eric