Fabricate a "holster" in the form of a snug fitting pannier with the bag extending high enough to ensure it can't tip out. Probably need a back liner that attaches to the rack top and bottom, and extends more than halfway up the horn. Or be much cooler, and do without the bag and use the back support and some mounting brackets of some kind so the entire horn is visible.
Me sure to make a bell cover in case of rain, and if you want, you can fly a banner from the top.
BTW- if you're used to doing so-callad "Roy Rogers" mounts and dismounts, you'll have to give it up
Thinking about it, trombones aren't that wide, so maybe you can attach brackets on the side of the top tube and carry it there running front to back between your legs. I've carried lots of long items that way, including (once) a pair of 8' 2x4s (never again).
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