A small radio has a small battery so i came up with this idea. A battery case for AA size batteries is available on the net . By removing all inside material with a Dremel tool enough space is created to hold 2 Li-ion batteries in parallel. Two thin brass strips can be used to solder the packs together resulting in a 2800 mA powerpack. Be sure that the two cells are fully loaded before soldering them in parallel. Look for Fuji NP60 batteies on ebay , they are simply glued together .Some foam rubber is needed to fill up the extra space between the pack and the cover.
Those who don't like to solder a battery can glue the 2 cells in anti parallel. You need to flip the pack if one cell is emty.