The drop-in speaker for Rapido U25B locomotives uses the UFIT-6735-RC1 speaker (this replaces the BOWS-RS3L, but sounds the same). The speaker uses a strap of tape over the top to mount. Please see the second photo for installation instructions.
 
The speaker comes fully assembled and tested.
 
Dimensions: 7mmH x 16mmW x 35mmL
 
VIDEO: https://youtu.be/mI6LbEf3eJ0