I started to get fed up, even when I got a spare charger for my phone it was still a pain to lug around, until I bored/inspired one day and decided to do something about it.
Step 1: First We See How It Works.
This is just so you can see how it came out, I will get into the specifics in the next steps.
Step 2: Preparing the Boombox.
I removed some stuff where the batteries can go in order to make some room and cut the USB cable to feed it through some holes I drilled from the front into the battery compartment. Then I spliced the other end back on after removing a good chunk to shorten it. Remember to leave yourself some wiggle room, you can always stuff the excess back into the body, but if you make it too short there is no going back.
I won't go into how I spliced the USB back together because it was really difficult working with those tinny wires and I was lucky enough to get it on the first try. You might be better off just using a short cable if you can find one, I just didn't want to make the holes any bigger than totally necessary to feed it through.
Step 3: Tie in the Power.
Start out by splitting the power cable far enough away from the end that plugs into(or is attached to) the boombox to reach the area where the phone charger is going to be hidden. You could just cut it there as ling as you attach both ends to the female plug, but I didn't want this to be a weak point where the cord could pull out .
After separating the two wires, use a wire stripper to cut the sheathing at both ends of each wire and use a sharp knife or exacto blade to split it from one break to the other so that it can be peeled like a banana while leaving the wires in tact.
Once you have the wires bare, slide the plug cover(the white part) over the cable and loop the bare wires over the terminals of the plug.
You can see in the third pic that I forgot to slide the plug cover over the cord first and had to re do it, but it gives you an idea of how to attach the wires.
You can see in the last pic that the fit is pretty tight but that's kind of good since it won't move around. So now you have a female plug inline on the power cord for the boombox to plug you USB charger and cable into, and it's all tucked away in the battery compartment.
BTW, I also had to cut a notch out of the battery door to make space for the power cable when the door is closed.
Step 4: Finally, Mold the Dock.
To prepare the phone I covered a spot on the back of the phone that would have made it impossible to remove from the mold by taping a dime over it. Then I covered the whole thing with plastic wrap and made a tiny slit in it so that I could plug in the power cord. The power cord should not be wrapped because it is going to be permanently cemented in place with the plumbers putty.
You can read the directions yourself, but basically you knead the stuff together until the colors blend and it gets warm and then you shape it into an elongated infinity sign. Press it onto the screw heads so that some gets in around them and then flatten it out a bit while leaving enough along the bottom edge to form the lip of the cradle.
Press the phone into the stuff and start to work it up over the bottom edge of the phone and around the USB power cord.
Mine didn't come out as nicely as I had hoped because I kneaded it for too long and it started to set up before I could really get the shape I wanted, but it works great and the cord is held firmly in place. The phone just sits right down on is and a connection is made, so now I just bring this thing to wherever I am working around the house and plug it in, plop my phone down and plug in the headphone jack and I've got endless hours of music while I work.
Step 5: Summary
~$1.50 for the plug
~$6 for the spare USB charger
A few months in it's working fine, but I got a new cover for my phone and it no longer fits, so I will either have to cut out the power plug and re-position it, or sand the cradle down where it doesn't fit.
If you are giving this project a go, I might suggest wrapping the device a bit more before molding it, and then lining the cradle with a felt pad or something. It won't give as exact a fit, and therefor, may ware the power port a bit more if you are moving the boombox around a lot with the device in it, but you will have some more wiggle room in the future.