The closer a horn "fits" the player the easier the upper end will be.
The mouthpiece gap and leadpipe are the primary components that need to be
considered when finding a horn that fits.
There are innumerable variations and each player is different as to what combo of gap/leadpipe (and to some extent, the bell)
will give them the easiest upper end.
Now, once you have an idea of what works for you, I will ditto what Gary said. Getting a Harreslon Summit with the options
that fit me made my upper end clearer, stronger and easier.