One tip about soldering is you need a decent iron. So many people I know say they're bad at soldering then you find out they were using some 30 year old thing that's about 100 watts and has a tip made from rusty steel half an inch thick. It's like most things. Having the proper tool for the job really helps.

I am going to make up a similar little box but mine has 5 AAA NiMH rechargeables in it with a low voltage drop out 5 volt regulator. It has a USB socket on one side and a pigtail on the other to go to the 770. I'll chop up the little cable that came with it for that assuming it has all four wires in it. If there is space in the box I am also putting in a small charging circuit so I can plug any 12 volt or so supply into it to recharge the batteries. Nothing fancy. Just charging at C/10 so I don't need a fancy circuit. Will take 15 hours or so to fully charge but I can leave it connected most of the time to keep it topped up. The batteries are 900mAH and so should easily power a USB keyboard for longer than the 770s battery life.

I'll take some pictures when I make it, probably tonight. I haven't rushed so far since my USB keyboard hasn't arrived yet!

Simon