Years ago I did 3 of these for myself, when loksound 3.5 was the norm. You can get a full size 21 pin in the tank, and I used a 20mm round speaker stuck against the back of the cab, I didn't think it looked too obtrusive at all. The cab just detaches with the speaker attached, I soldered bent wire to the speaker terminals which contacts 2 small pins I drilled into the cab floor. If I was having a go now I would use a zimo mx648 which is 10mm shorter than a full size decoder, and I am fairly confident you could fit the speaker in the tank as well. The problem is although I shoved as much lead shot into every space I could to compensate for the weights I removed from the tank, it is still a light model. When you compare it with the Peckett, they are similar size but the new model is a lot heavier due to its cast construction.