Congrats! Building stuff like this is way rewarding.
I built my 5x4 on similar lines - except I used mdf, cut a hole in the back to take a film holder, and put a lens in the front (pulling the box open and closed to focus). The total cost was about $35 including film holder and lens (no shutter).
I'd consider using the smaller box as the back half as this reduces light leaks (any light getting in hits the front rather then the film).