Man.. I built a custom boom box in a huge Dewalt DS300 tool box with 8" polk sub. Enormous, heavy, powerful, and sounded like absolute ass. Not even worth it, what a stupid project that I spent way too much time in building, debugging, and with great futility - trying to salvage.
I have a W-King 50w boom box now with 8,000mah battery built in. It charges over USB-C (though sadly not USB-PD) and is amazingly clear - and loud. No distortion to be heard at the upper levels of volume and it lasts forever. Can seriously hear every word of a song from hundreds of feet away. The bluetooth range is nuts, too. I think it's advertised as 60ft but I was triple that (line of sight across a meadow) with my phone and still playing this weekend. I can't find it on Amazon right now, but consider spending <$100 for an advertised 40-50w bluetooth speaker with decent reviews. So much less headache than building your own and hoping it sounds halfway decent.
These threads always have a couple cucks chime in to tell you that you shouldn't enjoy loud music when camping. I can walk around in my boxers, shoot guns all day, run the generator all night, and blast music as loud as I want. That freedom is why I go, and why I've spent years finding spots that I'm unlikely to see another soul for as long as I stay out there. I'll be damned if I put on headphones. Second I do that I bet I become cougar food.
Anyway, give amazon a whirl, find well rated non-shill reviewed products, make sure it's not a speaker released 8 years ago, and return it if it sucks.