Totally frustrated with carrying around dozens of rechargeable AAA cells just to keep one mp3 player happy, I did a "Macgyver".
1 * D-Cell Holder 1 * Piece of cable from ruined earphones
I just hard soldered it together and locked all the wires down with hot-melt glue. The battery goes in a pocket, the mp3 player clips to my collar and it's no more awkward than those mp3-interrupters that people use on cell-phones. In two years I've only used two D-Cells. I would call that a $ignificant $aving.