The current design for Симон is a 3.5" circle. I believe I've figured out how to get a speaker on it.
This speaker has a double-sided sticky foam pad on the bottom. The trouble is that in the center of the board, where you'd want the speaker to mount, there's a bunch of stuff that gets in the way - the audio circuitry in particular is there, and it's going to possibly overheat if it's buried under sticky foam tape.
The solution is a 3D printed little plastic "table." The table has two small risers spaced the same distance apart as the screw holes for mounting the battery holder. By inserting threaded split-wedge inserts in the risers they can act as the nuts for the screws that hold the battery holder in place on the back of the board. The opposite side of the surface from which the risers descend is a "table" upon which the speaker can be mounted. It will wind up being 3/16" up from the component side of the board, which still means the leads will reach the holes on the board, and the back of the "table top" is still 1/8" up from the component side, which is plenty of space to fit all of the stuff underneath.
I'll add the "speaker table" STL file to the files section of the project.