I did my 1936 pickup horn a while back and the first technique is patience.
Second technique is some painters masking tape. Place tape on one half of the horn bulb right next to the indentation. Place one side of the screen in the indentation opposite the tape side.
Now slightly bend the screen and try to slide it over the tape and into the indentation. Yes, there maybe some touch-up paint required, but it should be minimal.
Remember the first technique.
Hope this helps.