Yup, push moulding (crash moulding if you will) is the answer. Balsa is not the ideal medium though as it is a little soft and sharply defined angles can be a bit tricky, obechi or even scrap pine is better IMO. If you are unfamiliar with the technique there is a full explanation on the
Scale Model Aircraft in Plastic Card pages of
my WWIModeler site. If you adopt Michaels method of fixing the card to a couple of dowels, or strips of wood, the overcomplicated contraption of frames and bulldog clips shown below will be unnecessary. It's far simpler to place the male part of the mould on the bench, drop the heated and softened plastic over it, then push the female mould over that.