I've never tried this myself, so I don't know if it will work, so please proceed knowing that if you try what I'm about to suggest you may ruin your stuff!
Have you tried wiring a 12 volt DC power pack into the battery connections for a small boombox? The negative/ground should be wired to the outer sheath on the connector and the positive should be the lead/tip. Those power packs are good for around up to 1 amp at 120 volts.
Keep in mind that GENERALLY the center pin/contact is positive with the outer sleeve negative/ground, but this is not always the case.
Sounds like any car stereo with the right features would work. If you keep the volume low the current draw would be small and could be limited with a fuse by adjusting the value. No regulator would be needed unless you have a charger connected to your batteries so a car stereo capacitor might be a good idea. Unless you find a boombox that used 12v batteries internally you would damage it by connecting to 12v.