Hello I use earphone batteries size 675.
These 'air cells' lose power in a abt 3 weeks even if not used.
I suppose size also available in other chemistry types.
I use the Wumpus method minus the cardboard tube.
Just a screw and two nuts for a spacer in battery column.
Without the tube the batteries stay tight anyway because I
carefully filed down the screw to exact correct length.http://www.oldradioworld.de/srh436.htm
Would love to get an earphone plug, thanks!