From Plano's FAQ page:
Q: Can I get extra dividers for my StowAway® utility boxes?
A: Plano may be able to provide extra dividers on certain models of utility boxes. If you are interested in finding out if extra dividers are available for your utility box, send an email to [email protected] and include in the email: model number of the organizer (stamped on the bottom) and the number of dividers you need, along with your name, address and phone number. If available, small quantities for your own personal use will be provided at no charge. Larger quantities or for business or resale will be available for a fee plus shipping and may be ordered via credit card from our Consumer Customer care department at 800-226-9868, M-F 7:30a.m.-4:00p.m. (central time).
But now you can manufacture your own, in any color, material, and quantity you like! As with some of my other designs, this isn't strictly a remix, but I like to credit those who came before me.
Autodesk Fusion 360 source: http://a360.co/2njqoJv
I print these in PETG at 0.18 mm layer-height. The chamfered edge on the underside aids in prying them off the print bed.