i found some good direction in this pdf (page 11)
"The vertical slit determines the horizontal focal length and vice-versa.
The width of the opening of the slits is the same as the ideal pinhole diameter. Remember the front and back slits are at different distances/focal lengths.
The length of the slit is determined by were it is in relation to the opposing slit and is determined easiest by looking."
and it states in the directions of a 4x5 pinhole for front panel options: (c)
"a) Straight pinhole designs, zone plates and hyperfocal cameras
b) Multiple holes, overlapped and separated
c) Double slit and double zone
Two front panels needed [pinhole side]. The second panel is
recessed into the interior of the box the desired amount for the
second focal length. The measurements of the box will be based on
the longest focal length used."