The case I used:
Cable to buy:
Really, any micro-usb cable should work, but I like the right angle on this one.
As near as I can tell, any of them will work, but this one is really cheap.
Total cost of all three components is less than $20.
Destroy the plastic housing on the powermat reciever. When you get done, you should be left with something that looks like this:
Obviously, this one has a usb tip already soldered on to it. We'll get there.
Step 2: Clean all of the sticky stuff off of the powermat with an exacto knife or rubbing alcohol, then remove any leads from the four terminals across the bottom. (There should only be leads on pad 1 and 4).
Step 3: Cut your usb cable in half, leaving around an inch-and-a-half of wire on the end with the micro-usb connector. You can throw away the other half.
Step 4: Now for the hard part. Strip back the black exterior about a half inch, and trim away the braided shielding. Cut all of the wires off EXCEPT the red and black wires. These are your ground and your lead wires. Then VERY CAREFULLY strip them. I would recommend just scoring the material and pulling rather than trying to cut it and remove it without damaging the wires inside.
Step 5: Now position the unit with the black magnet upwards and the four terminals towards you. Solder the red lead wire to the far left terminal, and the black lead wire to the far right.
Step 6: Place a piece of electrical tape over the soldered wires to reinforce them.
Step 7: Trim away as much of the plug end as you feel comfortable with. If you cut the perforated part off, you can get the wire to run straight down and fit much more neatly.
Step 8: Place the unit in the case magnet-side up, and push the plug through the usb port hole in the bottom of the case. It should now look like this:
Step 9: Put your phone in the case, and plug the usb tip in. Your phone will now charge on a Powermat charger.
Step 10: Profit???
Final product in case: