After having seen a few in the wild and having one crummy proof-of-concept build, I ordered parts for and built a brand new radio cooler in the fall of 2013. It started with two speakers and grew to four and has since been used hundreds of times between tailgates, beaches, and poolsides. It lasts longer, is way louder, and is much more hardy than a lot of store-bought speaker systems. I built mine to be as water resistant as possible; I used all marine audio equipment and tried to make decent seals around the speakers and lid. It can get rained on or be thrown into a river float and still provide powerful loud sound for tens of hours.
My radio cooler is built from mostly inexpensive components and was constructed such that any component can be swapped out quite easily. For this build, I used:
So all together it costs around $275 to build one of these give or take some depending on what kind of battery you use. You can also use any type of cooler that you feel comfortable mounting the amplifier and speakers in.
Putting one of these together is much easier than it looks. The battery essentially hooks straight to the amplifier and the amplifier straight to the speakers. After that, you just need to wire in a toggle switch from amplifier (+) to the amplifier toggle post. Plug in the RCA adapters in connecting the amp to your phone (or other aux/RCA output). Turn it on and start jamming!
A common question I get about this cooler is whether or not it can actually hold drinks. The answer for my build is no. In order to maintain relative dryness inside the cooler, I try not to invite liquids (except perhaps sunscreen) inside.
I measured and cut the holes in the cooler using a reciprocating saw. Speaker and amplifier components were mounted using bolts with wingnuts through pre-drilled holes. I soldered the wires to the speakers but this is not required. All amplifier connections are using terminal connectors and the battery rests on the bottom with small brackets preventing it from sliding. There is a fair bit of storage space inside.