I purchased 12' of .5" diameter tubing, a small copper pipe converter, and a set of funnels. These became the main tubing, mouthpiece, and bell of the horn, respectively. By simply sliding them into one another, I created a simple horn.
The interesting thing about this horn is that it's a natural horn: it can only play the overtone series, but no other notes. If the horn had valves to change the length of tubing, it could play notes outside of this series.
I have never played horn before, but I went ahead and made a video of me trying. Hopefully, I'll get better soon!