Hornby 8F with Zimo Sound
First remove the weight and prise off the plastic supports. using a razor saw cut down the rear weight fixing pillar flush. Drill some holes in front and behind the centre wheels. Cement in two small fillets of 1mm plastikard to seal the bottom of what will be the output chamber for the speaker - a couple of nicks in the corners will allow the pickup wires to pass through.
Using two layers of 1mm plastikard build up a framework
then cut a baffle/top with a nice big hole. Note slot for the rear tender top mounting cut to match where the old weight finished
Bond in place
fit speaker using "black tak"
some more black tak around the inner edge of the tender top seals and prevents vibration
Feed two fine black wires through to the loco for the speaker connections and Robert is your Mother's brother