The battery is held in place by an elastic band looked over 3 clips. Two at the back and one between the motor sockets.

  1. Make the 3 battery holder clips from metal paper clips by cutting off the inner loop of the paper clip, then bending the ends in towards each other and then folding the end over to make a hook.
  2. Solder the ends of two of them into the two holes at the edge of the board beside resistors R2 and R5

  3. Solder the third one into the 2 holes at the other side of the battery space which is between the two sets of motor connections
  4. A small elastic band is looped over all 3 hooks as shown to hold the battery in place.