VetPsychWars

The Buescher 88-D for the trumpet and 99-D for the cornet are the same size cup. The diameter of that cup is reasonably close to a Bach 10-1/2, so you might try a 10-1/2C or the equivalent Schilke or a dozen others. The rim is somewhat flattish, so that would be a factor to consider.

Something to note on the Buescher mouthpieces is that the high point of the rim is to the outside of the cup rather than the inside, so it might feel larger than it really is to you.

As an interesting aside, the cups were specified A, B, C, D, and E, where A was the deepest and E was the shallowest. They described the D, therefore, as medium shallow.

If you think the Duo-Cup feature is helping you out, then you might also want to try a Parduba 5, which should be about the same diameter and has the dual cup feature as well (though it's implemented a bit differently).

As for an adapter, normally a decent idea, but the Buescher 99-D is a short-shank piece and the intonation is likely to be goofy.

If you have any more questions about these pieces, which I use in my Buescher instruments, let me know and I'll try to answer them. Which cornet did you get, by the way? There's a model number at the bottom of the second valve casing.

Tom