Another tip:
some mouthpieces have a form with edges like this:

 _____ 
/     \
\_____/

and some are more rounded, more like ().

(sorry, not easy with just characters and some HTML/CSS to draw)

And some (like the Guinness one) have edges on top and are round on the bottom.

I don't like the kind with the edges - they are uncomfortable at the lips.

I took a piece of sandpaper and removed the edges from the Guinness whistle - that doesn't change the sound but is much more comfortable to play.