Back when the Bachmann HO scale Shay was first released, SoundTraxx came out with a loco-specific decoder that was plug-n-play. The speaker provided by them at the time was only a 3/4 inch round one, and even it barely fit.
I think your only hope is to go with a smaller speaker, but the sound quality and volume really suffers because of the small size. My Shays are barely audible even if no other sound locomotives are on the layout.