Inspired by Tom's (toms3d) recent video, I thought I'd post this design which I have had sitting around for a while.
It is folded double horn with integrated phase plug, intended to integrate with a small speaker ~40mm in diameter.
There are mounting holes 54mm apart, but you don't need to use them if you are experimenting, you can just hold it over the speaker provided it has a suitably flat surface.
It prints in one piece without supports, provided you have good bridging.
The thing works, bass comes out of the longer folded horn, and treble comes out of the small horn in the front. Sound quality probably needs work, but that's what the Fusion 360 file is for. The timeline in the file is a mess, but you'll be able to see how it went together, and you should be able to edit elements of the horn profile without breaking it.