You could just buy a darkroom vent but essentially you just need to "light
trap" the hole.  The idea is to construct a simple little "maze" out of
light absorbing materials, the air just flows through the maze whilst the
light is soaked up.  Might even be able to do it out of a couple of ABS
plumbing fittings street "ells" glued in a "S" shape would do it.

On the other hand, if I wanted to get fancy and really over-engineer the
thing and had a 12-volt power source (solar car battery charger?) I'd hook
up an 80mm fan from a computer to the vent.

On the other, other hand (am I up to three hands yet?) I'd probably just use
a changing bag.

zachary spake thus:

So what's the theory of operation behind this "portable darkroom"? Is
the idea to be able to make prints out in the field? or just process
film? If so, why? Wouldn't it be easier just to transport the exposed
film back to home base and process it there?

Just curious.

Are they ventilation holes or different sized holes for having
different sections for images on the pinhole camera?  You could
theoretically load anywhere dark and have it wrapped in a light
blocking bag or any container that is totally light blocking for
transport to the place you are using it?  Just a thought, no expert
here.  I have done pinholes, I don't understand why you would need
ventilation in the box though, honestly.  I made a pinhole with 4
different holes but they were used to make 4 images on one sheet of
paper - sectioned off into quadrants inside the box with foam core
"walls".  Good luck!

  Unless this is a very large pinhole camera you can use a
plain changing bag. There is no ventillation but, unless you
are going to work in it for very long periods of time, none
is needed. Changing bags are readily available both new and
used. One can get very elaborate chaging bags with frames
inside from Calumet and others. These are sometimes
convenient for large size projects.

  As a general answer to ventillation, one can make
openings with a labrynth or maze arrangement around them
with the insides painted flat black. These will block light
pretty effectively.