How I am going to make my battery pack for my new Raspberry Pi

I've seen a few forum posts about battery packs.

Above is pictured the "case". I intend to do some dremel work and a lot of sanding, but the dimensions fit almost perfectly, and it (most importantly) has a nice bay at the bottom for batteries.

So, my plan of attack is pretty simple. I have some electronics experience, though I will admit it was mostly along the lines of crimping ethernet cables and plugging my cables in the right places when I build computers.

• Find an extra USB charger lying around. Take it apart.
• Hook up a power switch to the battery bay's wires.
• Hook up the battery bay (via the power switch) to the transformer in the charger
• Plug microUSB into charger's USB port
• Stick batteries in the bay. 
• Plug microUSB into Pi
• Turn on Pi!

I'm going to try to get some more experienced electrically-minded people to have a look at this before I implement it. Feel free to leave a comment if you have any ideas/knowledge you are willing to share.