anything thats really deep, im origonaly a cornet player so i used to really like the dark mellow sorta sound, try a bach 2-1/4C or a schilkie 20# they were my 2 main mouthpieces there really deep which give a nice warm dark sound but the high register will take some practice, or any deep V shaped ones, or grab a cornet mouthpiece and get a cornet to trumpet adapter, there not to epensive
If you want sound close to a flugel there are a variety of manufacturers who make mouthpieces with a flugel type cup on a trumpet shank (Warburton, Curry, Monette, Wedge, to name a few).
I use a 1.5C, or a 1C, they are both pretty large and give me a very nice open dark sound...you might even wanna try a Bach mega-tone or a CKB since the added weight (SUPPOSEDLY) darkens the sound even further. (I however can hear no difference)
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