I found this piece which seems to be quite handy for travelling. Not does it only allows a unit to give different voltage outputs, i.e. (as I understand it) no matter what voltage 4 separate batteries give (above a certain voltage level) one can chose from the different modes (3.6, 4.5, 5 and 6V).
It also allwos direct recharge of the batteries in use, handy since one can use a single unit instead of two separate. It also comes with USB.
http://www.semsons.com/powerbank.html
jburtonca, does your homemade box work just as well as Big Dīs maplin box? I.e. does it allow you to recharge the ihp-120 2.5 times? Does it take both NiCa and Alkaline?