You don't need a new box, you need a hacksaw blade for the box.

Thought of that. The blade size for this box does not match standard
hacksaw blades. Is a bigger (longer and wider) hacksaw blade made?

Answering my own question, apparently the Stanley miter box saw will take
Jorgensen 550-mm blades, which are slightly shorter (about 1 cm). If
necessary I will modify the saw just a bit. Jorgensen has a full line of
blades from 14 tpi (soft wood) to 32 tpi (steel).

Found them at www.adjustableclamp.com.