So, here's the parts list:
- TA2024 Amplifier:
This is a highly efficient amplifier that will make the most out of your battery, and it's incredibly cheap as well. There are hundreds of different brands and casings that all contain the exact same "Tripath TA2024" chip. I go for the cheap bare motherboard. It is 15 watts x 2 at 4 ohms. That might not sound like much, but with the right speakers this will rival or even outperform the stereo in your car.
I like the Sure Electronics version
http://www.parts-express.com/pe/showdet ... er=320-308
But you can find very similar units on ebay for $9.00 shipped, like the MKII Tripath TA2024
They will all sound and perform nearly identically as they all share that same crucial chip which does all the work. The efficiency of this chip is crucial, you will notice there is no heatsink, this amplifier creates almost no heat which is the key to a powerful battery powered boombox. If you use an amp that creates waste heat, all your battery power is wasted in that heat.
- 12 Volt Rechargeable battery:
Now we want a small, lightweight battery, luckily batteries are evolving pretty fast these days. Thanks to all the manufacturing in China and eBay you can get really great technology cheaply. If you search ebay for "12v battery 6800mAH" You will find a ton of these batteries that are about the size of a pack of cards and not much heavier. They have kept the boombox going for a whole weekend for me depending on usage and volume levels.
For around $20 you get a lightweight and compact 12 volt battery that is nearly 7 amp hours, and includes a 120v charger.
- The Speakers:
I used the Lanzar VX830 8" 3-way speakers. But any 4 ohm speakers will work.
http://www.amazon.com/Lanzar-VX830-8-In ... B0007L8BBM
The only downside to these speakers is their weight, at 11 pounds for the pair, the boombox is heavy, everything else combined is probably 2 or 3 pounds total.
I built a boombox before this one with 5" speakers that were 8 ohms, it was very disappointing. The 8" Lanzar speakers are 4 ohms and 90 decibels per watt, in other words they output 90 decibels of sound at 1 watt of power. The 5" speakers were closer to 80 db/watt, since decibels are logarithmic, that's approximately half the volume at 1 watt. Further they were 8 ohms, which caused the wattage output by the amplifier to drop by about half. All that to say that the speakers you choose really matter. The boombox with the 5" speakers was 1/4 as loud as the 8" speakers.
Generally speaking, larger speakers are more efficient, 6x9 speakers would work well, but are a funny shape. 6.5" speakers would work as well, but I wouldn't go any smaller than that and look for that db/watt measurement. That really is the key!
I did a lot of shopping and the Lanzar VX830 had all the right features at a great price.
These 6x9s look lightweight, have a good protective grill 92db/w, pretty much perfect, just that odd shape
http://www.parts-express.com/pe/showdet ... er=269-185
Finally, one last idea which takes out a lot of the woodworking fun, but is very practical and inexpensive is to get this prebuilt enclosure with the drivers already in it.
Acoustic Audio AA-8XT
http://www.thedeepdiscount.com/p-6938-a ... eaker.aspx
Also on eBay, you get the enclosure and drivers for under $60 shipped.
In addition to those 3 primary components, you will need a couple feet of speaker wire, a headphone jack, a power switch and of course some wood, glue, and a handle. The power switch is a rocker switch with a built-in LED so you can tell when it is on.
Build your box however you like, use your imagination. BUT, build it sturdy and stout. I used cabinet grade 1/2" plywood for the front and back, the top and sides are a 1x5 (3/4"x4.5") that I routed 1/2" grooves in the front and back to lock the plywood faces in assembled with clamps and wood glue. After it was dry I using construction adhesive along all the inside seams to seal it up tight and add more strength. Finally there is a block of wood in the middle between the front and back face. This thing can get pretty loud, and I really wanted to lock everything down and stop any buzzing. It's overbuilt, but it sounds great!
Please feel free to ask any questions and I'll fill in anything I missed.
Hey! You stole that line from me. DH was proud of himself when he came up with it. At one time I used that for my sig line. It's really true we ALWAYS get stuck next to screaming dogs and barking kids. Use the quote in good health. 
and I felt it describes the cacophony you encounter in all too many camp grounds. I remember a discussion on this forum of how do you hint to some one that their portable noise is too much and one reply was to play bag pipe music. Compass Rose has a stereo and Sirius satellite receiver but the speakers are inside and with the galley hatch closed you can't hear the radio.
