I ran across this blog years ago. I even linked it to my website.
For me, we use plain white rice in the bulk bags. Changing it out
as it gets dirty or starts showing yellowing.
http://blog.ferretsnorth.org/2011/04/dig-boxes.html

One piece of advice, either make the entrance hole on the top or make
sure the tube leading in is higher than you rice level. Mine enjoy a
longer tub than a deeper tub. I also find that some of mine enjoy the
harder beans or even my wood stove pellet clean bin for an extra long
scratch.

Thanks 
Kathy of Sammy's Clubhouse Spokane Ferret Rescue and Education
Main website: https://sites.google.com/site/sammysclubhouse
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[Posted in FML 8011]