Now that I have the display and controls in place I've been working on the positioning of the speaker and amplifier. The ideal place for the speaker would be on the front so that sound is directed straight at the player. I don't quite have enough space for that and I don't want to increase the overall dimensions. I do however have just enough space to accommodate the speaker along the bottom with the amplifier (this is the one I am using https://www.adafruit.com/product/2130) alongside it. 

To test this out, I've printed another test piece of the bottom front of the case. I've increased the thickness of the bottom (without increasing the overall dimensions) to create recesses for the speaker and the amp which are held in place by a small clip attached to perfboard for the controls. This is what it looks like:

Here you are looking at the bottom of the case from the inside. The long screws with nuts are there because I didn't have any of the right length handy. I'll cut some down for the final version. This is what it looks like from the outside:

Finally, here is a side-by-side comparison of the front test piece with the display and controls fitted (but not wired up) with this one. I've also dropped a Teensy on there to show where it will go:

I also wired the speaker and the amp up to check that they work like this and that the wiring fits. My tests were successful, so next stop: Teensy.