Here is a remix/remodel of fifo_152's bluetooth speaker, made to fit a print bed of a Prusa i3 MK2. Thank you for the model fifo!
The material I used was PETG, although other durable materials with less flexibility would be preferrable.
If I recall correctly, I printed the chassis (biggest part) with 3-4 perimeters and reasonably high infill (to make a solid wall with no resonance), maybe even 40%.
The back and front panels had 4-5 top/bottom solid layers to prevent, again, rattle from the speakers. Infill was 20-30%.
The decorative rims which keep the cloth suspended were printed at a low layer height (even 0.05mm) to get a good and clean looking result. Print with the curved, "out-facing" part pointing upwards with supports, they may appear upside down in the preview and in Slic3r. The small fillets on the start of the print don't need supports.
Inside the case there are quite a few screw holes for mounting various boards as well as the battery pack. You may want to customise these to fit your own parts.
The speakers I used were Visaton FRS-8M, although I'm not entirely pleased with their bass reproduction in a small casing like this. A software equalizer helps a little.
On the back plate I glued a pair of passive radiators to improve the sound on lower frequencies. There are holes for a certain DC jack and a 3.5mm connector.
On the front plate there is a hole for a 5mm LED.
As batteries, I used 10 2600mAh LG 18650's wired 5S 2P. For balancing and protection I used a readily available 21V board from aliexpress.com. An external 21V charger charges through the DC jack.
The Bluetooth chip is KRC-86B v4.0, however I have a few complaints about it: The volume control buttons are reversed, as in holding the button down changes volume, pressing it once skips a track. This causes skips by accident. Also the volume max alert is obnoxiously loud. Remember to ground the chip properly to prevent noise.
I then used a step-down (buck) converter to reduce the voltage from 21V to 5V for the BT chip.
Amplifier I chose is WONDOM AA-AB32472 with 2x30W RMS output, available in sure electronics and eBay.
Finally, on the top there are four 16mm buttons to power on, play/pause, vol+ and vol-. Make sure your buttons are either short enough not to hit the speakers on the inside, or move the holes for the buttons a bit backwards.