I am trying to locate a solar-powered AA/AAA battery recharger that will fit on top of a backpack lid. The space allowed is a maximum of 12" x 6" (300mm x 150mm)
Of course it must be reasonably lightweight!

Please advise if you know of such a device?

Goal Zero Guide 10 Plus Mobile Kit is 10.25 X 6.25 deployed and comes in at 15 oz with 4 AA's. Bonus is you can use the Battery pack alone in your urban setup (briefcase or on a plane).

+1 on the Goal Zero. I have it, it works great and there is an option to use a 12V charger cable. The GZ comes with a set of cables that plug into the USB port. It will slowly trickle charge a device that way, but it works best if you plug the battery pack into it, then plug your device into the battery pack. Really won't charge your electronics on the go, but will recharge them via the battery pack. I got mine on Ebay for way less than retail, but shop around.