Thanks for the suggestions: I have thought about a solar charger but I don't know how well it will work on a dark rainy day, which is mainly when I will need it. The battery packs with a lithium rechargable battery would work, but I think one that uses replacable AA batteries would be more useful. Building one myself is a nice idea, 4x NiMH batteries would make 4.8V which might be close enough to 5V, although I'm a little nervous about damaging my reader if I get it wrong :-(
These look almost ideal:
http://www.japantrendshop.com/eneloo...bs-p-1171.html
http://www.japantrendshop.com/sanyo-...er-p-1234.html
The second one can use a USB source to charge the AA batteries as well as use the AA batteries to charge a USB device, which sounds neat. I just wonder if anyone has actually used one of these or something similar with the Kobo readers? Some devices seem fussier about the power source than others.