I made some updates to the airhorn up on instructables:
Works really well, makes a lot of noise
This is the big hole version as suggested at the bottom of the article. Includes all the pieces to make it possible. I cut it out of 1/8th in birch plywood.
Don’t have a photo of mine either, but the instructables article does have a photo and it also helpfully lists all the instructions on how to make it.