These lids seem to be hit and miss. The lid should, in theory, fit any of the boxes with the 18x12 dimensions - no matter how deep the box is. But we have discovered that they don't always fit the 6" and 9" boxes perfectly. Some fit better on the shallower boxes and some fit better on the deeper ones. We generally don't need a TIGHT fit, so when the lid won't fully snap, we just invert it and it closes up that way.
Thank you for your feedback! We’re sorry this product hasn't worked consistently for you. For recommendations, please contact our Customer Solutions team.