Quantity   Component name
1 × Chinese boat/car amp These things are all over Amazon. Just don't believe the wattage, because none of them are actually 500W. Hell, mine is lucky to have 5. Some of them even decode MP3 from SD cards.
2 × 3.5" car speakers This is an uncommon size, especially if you want *speakers* and not just tweeters. Try looking in 80s German cars with small doors.
1 × Plastic Skil toolbox on Canadian Tire closeout sale It doesn't have to be Skil. I just like to say it. Skil, skil, skil...
1 × PC ATX PSU I found a low-profile fanless one but you can probably adapt any PC ATX PSU to serve this duty. Just short the green pin to ground and the PSU will always stay on while it's plugged in. You could also, you know, add a switch, but where's the fun in that?
1 × Standalone Bluetooth speaker Just crack the case open. This one was held together with screws but others are held together with glue. You'd be surprised just how little is inside these things.
1 × 3.5 inch hole saw bit To make holes in the lid for your speakers. Ideally you also want a drill to go with these, but I'm not going to stand in your way if your wrist can spin 18,000 RPM.