Reddit mentions of Sackorange 6 pcs 3 x 3.7V Battery Holder,18650 Battery Storage Case Plastic Box Holder Leads with 3 Slots for 6" Wire Leads(6 pcs 3x18650)

Ok, you have about ~3.5 liters of internal volume to work with. It's pretty tight for any more surface area than a 3" driver or equivalent. I'm going to assume that you're okay with a ported box over a sealed box since you'll get a lot more sound out of the small form factor.

Drive-wise, there are 3 easy options:

• 1x full range 3" like the AuraSound NS3-193-8A which would go into the box with a 1" diameter port (or any shape of equivalent cross sectional surface area), 6" long, which would tune the box to about 50 Hz. This design gets you the most dB's for the least $$, as far as I can tell. The port will add the low-end where you want it for music. If you want to go completely sealed, you lose an octave of bass.
  
  
• 2x 2" speakers like the Dayton ND-65-4 with a port tuned to 70 Hz, which would be a 1.25" port 5.5" long. This option is simpler in terms of bluetooth amps.
  
  
• 2x pre-built speaker module such as the Tang Band T2-2181SA (pricey, but simple)
  
  
  Then you'll need a 2x50W class D amp with bluetooth pre-attached. There are tons of these all over amazon; my favorite chipset is the TDA7492 or the TPA3116D2. Wondom, Dayton, Drok, Sure Electronics all make decent versions.
  
  For power source, if you want batteries that's a bit of an added cost, otherwise, you just need a 24V power supply rated in the vicinity of the nominal output power of the amps (75W will be more than enough). Otherwise you'll need a 24V battery pack, which you can make out of 6x 18650 cells (I linked some expensive ones because there are a lot of fakes on Amazon, there are cheaper "protected 18650" cells out there) and wire them in series with 18650 battery holders and you'll need a 6S li-ion charger. You can always opt for a 12V system for simplicity; you'll need the same bits but just 3-Series instead of 6.
  
  If you want a cool switch, there are a billion options
  
  The speakers range from be $15-$60, and power supply (battery or wired) will be an extra $20-$60 depending on what options you choose, making it $65- $150 (with the amps) in total, for the outlined options. And that's about all you really need.
  
  If you want to get fancy, you can add some inductors to high-pass filter the amplifier output in order to prevent the speakers from bottoming out, but that has to be calculated on a case-by-case basis. Let me know if any questions.
  
  Edited for clarity

3S 18650 Battery holder

Balanced charger for above

Note that the charger board has 4 connections to the batteries and 2 more for the 12V in/out (6 total). Like this.

You'll want a 12.6 v supply for this (not 12v - 12V isn't enough to charge this)

You can tie the charger, battery pack and heater coil all together. Make the charger so it unplugs (obviously). When you have the charger plugged in, it'll charge + give a boost to the heater. Unplugged, the heater will run on the batteries.

This battery configuration will only put out about 2-3 amps (I forget), so heating is a little slower than the 6 amps the heater really wants. The difference is like 15 vs 18 seconds.

For some good glass tubes (for inside the coil), search eBay for "Cloupor Cloutank M3 Glass"

The induction heater? Made it. It’s just a $10.49 part off Amazon, a switch, and 3 18650 batteries.

The guy on FC selling that skeletor kit has to be laughing to the bank. He’s quadrupling his money on those things.

edit: Amazon link for heater in case anyone wants to buy one. Just hook your 3 18650 batteries to it with a holder, put a switch in-between, and you're good to go. Those are the exact parts the skeletor kit uses, he's just making it look tidy by soldering things rather than just sticking the wires in the plugs and screwing them tight.

In that case, I'd go for something like this. Replace the existing wires with 18 gauge and it should work well. Also get something like this for charging and protection.

[Battery holder for three 18650 batteries] (https://www.amazon.com/Sackorange-Battery-Storage-Plastic-3x18650/dp/B071ZQ2X2T/ref=sr_1_12/140-5364689-5700410?ie=UTF8&qid=1517732407&sr=8-12&keywords=18650+battery+holder)

Just buy one of these heaters plus a battery holder then add a momentary switch.

edit: If you want to be super cheap you could make a battery pack, skip the holder and switch, and just plug the battery in to the heater. Not that I'd recommend that, but if you want real cheap that's an option.