Just go get an old car tape deck from the junk yard. A lot of them will have a built in amp, RCA output, and run on 12v, and is very compact. My garage stereo f

Just go get an old car tape deck from the junk yard. A lot of them will have a built in amp, RCA output, and run on 12v, and is very compact. My garage stereo for years was an old Blaupunkt CD player with an aux in. I mounted it on the bottom of a shelf and it took up no space. I just used an old wall wart to feed it 12v. It put out lots of volume, even though the wall wart was like 12v 1a. This whole setup will cost you like $5-10. I've seen yards that have piles of aftermarket decks just sitting on a shelf. You'll be able to find even more tape decks if you look for OEM units.