Follow up:
Balsa seemed too flexible, so I cut a circuit board (blank, prototyping board with pre-drilled holes on a grid) and it worked out fine. On the first attempt, I managed to crack the pc board while cutting out the servo holes, but on rev. 2 I spaced out the servos so they weren't so close and there was more material between the two holes. Only problem was that the board is thicker than the original receiver's, so it was hard to fit into the mounting bracket at the bottom.