You can buy a heat sealed 8 cell flat pack that fits right in there. Slightly smaller than the tray that comes with the radio, but you can take up the slack with some foam rubber.
Even Radio Shack sells them. 1600mAH.
For the Airtronics M11:
Maybe you have this type:
The ones above are only 400mAH, but I posted the pics to show you that sealed packs can take the place of the holders.