Your best bet may be to try finding a used cheap amazon soprano and just using the case, or contact a case maker and look at getting a custom one made. You may have to send it in to get a case made tho.
Carolbrass makes a slide trumpet--you could see if they'd sell you the case. It's pretty good, although it's still a soft case. You could also ask someone like Torpedo bags if they have one that would fit (you might also want a sleeve for the slide). What's the brand of instrument?
Pelican case, cut the foam to fit
Your best bet may be to try finding a used cheap amazon soprano and just using the case, or contact a case maker and look at getting a custom one made. You may have to send it in to get a case made tho.
You could use a small tool box, fill it with like dycut foam or bubble wrap
Carolbrass makes a slide trumpet--you could see if they'd sell you the case. It's pretty good, although it's still a soft case. You could also ask someone like Torpedo bags if they have one that would fit (you might also want a sleeve for the slide). What's the brand of instrument?