I am currently using a GR 66FD, its a flugal horn moutpiece with an adapted trumpet shank. It fight in every trumpet I have ever tried, and It fits beautifully in my mellophones (both F and G)
It has a larger bore, but a very comfortable rim.
There are alot of sizes, so be careful which one to get. I had the luck to try out every size due to a dealer that I knew, I just picked the best suited.
Dont crack it till you try it, the overall tone got darker, and the overall depth of sound increased!