found this but i don't think it explains enough to build one, didn't read everything though Homemade air horns
the good thing about making them your self is you can tune the to sound you want, by changing out material type and thickness as well as different PSI. Just remember the faster the diagram moves the higher pitch it will have and slow it moves the low pitch it will have.
ok so last night i couldn't really find anything except for trainhornforums.com and the site is down. i believe that once that site is working ill be able to find free plans. Darkkshadow, i think it is possible to build off that site you found, just need someone with a small idea on how it goes together. now off to work
YouTube - PVC Leslie 200 Train Locomotive Ship Air Horn found this one, if you can blow it indoors with no ear protection, they are no where near loud enough