I would be afraid to use popcorn in a digging box, because the popcorn has
hulls on it that are tough, and could clog up their digestive tract.  But
I have an alternative that you might be able to use.
 
A while ago, I purchased an article from ebay that came packed in the
edible packing pasta.  It is made of cornstarch and water that is mixed up
into a froth and extruded.  You can tell you have the edible kind because
it is an off white color (almost a little greyish tan), and if you bite
into it, it literally melts in your mouth.  It tastes like an incredibly
bland corn tortilla chip.  The bland taste is probably why I saw it on a
news show being served with salsa.
 
I put it in a box, added ferrets, and watched it explode.  The stuff goes
flying while they dig, but they have a great time.  Even if they chew on
it, it won't hurt them.  (They haven't actually eaten any, but it won't
cause a blockage.) You might be able to get some from your local packaging
company.  It's well worth the effort to get it.
 
Melodyt and the elven eleven
[Posted in FML issue 2899]