Did the elastic band fixing your speaker to the chassis break long ago? Try printing speaker-holder.stl. This fits right over your speaker and snaps onto the four tabs built into the chassis. You can use OpenSCAD to modify the .scad source file in case your speaker dimensions are a little different (this relies on accurate tolerances for snapping action). The way it should look after being installed is shown in the photo. I used PLA+ for this particular print.
