How to carry a trumpet for long trips

I'd suggest a top-of-rack bag for your instrument, because its more aero when the load is in-line with the rider. If there's no bag big enough, use a series of bungee cords to securely strap the case to the top of the rack, lengthwise. Feed a cord under the case's handle too. If this becomes a regular thing, you can even make a "tray" permanently secured to the top of the rack, exactly the right size for your case, with lips/sides to stop it sliding out.

You could use panniers / saddle bag on the side of your rack, but that tends to stick out into the wind a big more. There's also the increased risk of damage being stuck out. But a pair of panniers lets you carry other stuff on the other side, and still use a top rack bag/tray.

Carrying the trumpet on your back is a workable solution too, but it increases the sweatiness of your back, and can exacerbate damage and injury in the event of an accident so its not ideal.

Carrying it inside the frame triangle, or above the top tube is probably not going to fit, and will interfere with your legs while pedalling.

A front rack might be a good idea too - you can see it while riding.

A trumpet is not really large enough to need a trailer, so that can wait until you upsize :-)

---

Whatever you choose, also consider waterproofness. Most instrument cases have a velvet / velveteen liner fabric which doesn't like getting wet. If you have to ride in the rain, consider a "dry sack" large enough to hold the whole case and seal up. Good for a towel and dry clothes too. This would also help prevent marking of your case by vibrations against the rack.

This is a great question on the best way to carry an awkward, bulky load that isn't very heavy (in the greater scheme of things).

The answer i expect noone else to give is to look at a front carrier option. This is best achieved with a small front wheel to keep weight low and not to hinder vision.

For instance, Crust make the Clydesdale fork to convert a standard bike to a cargo bike. See https://crustbikes.com/collections/forks/products/clydesdale-cargo-fork

The idea is that it gives you a great cargo carrying ability for trumpet-sized loads while keeping the handling and ease of getting on and off your current bike the same as you're used to.

You could also consider a different frame that allows for this type of option eg the Soma Tradesman or other similar design based on the classic 'butchers' bike''. https://www.somafab.com/archives/product/tradesman-cargo-frame-set

Obviously these aren't the cheapest options, but if you do alot of cycling with the trumpet it will quickly become the most practical and convenient.

And here is the alternative answer. Yes, I have done this before. I played in the band during high school and sometimes I would ride my bike instead of driving my car way back in the dark ages of the low 80's. The band teacher would let me store my expensive racing bicycle in the band room but one time I forgot that I would need to take trumpet home and ended up holding it with one arm while steering with the other arm. All while wearing cycling shoes and again, an expensive racing bicycle. Yeah, I did end up hitting a car with the expensive trumpet while riding on my expensive racing bicycle. It did help that I hit a car of a friend of mine who was driving a POS car [just like my POS car] and didn't worry about it. Short story - there are things that a car is made for - this is one of them. You are dealing with one of two different expensive things: an expensive trumpet or a car which is once you configure insurance and the car itself will be expensive. Take the car. Ignore the POS bicycles that have baskets, there are times that you just need a car.

I would recommend carrying your trumpet on your back instead of putting it on a rack on the frame, unless you have it adequately insulated against vibration, because that is going to be your trumpet killer. Once you start adding a bunch of padding which you should, it’s going to take some weight and space. Carrying it on your back is going to be the best protection. Other than that some suspension system that keeps it from rattling, the valves loosening, small scrapes and dents, and bent pieces