With terminal tackle, there is often a lot of wasted space in flat tackle boxes.  Several of the boxes pictured were near full and collapsed down to about half full using this tackle storage system.
Using only the dividers provided by the manufacturer, hooks and other small items tend to move from one compartment to another by going over or under the dividers.  The closed side trays provided here prevent migration under the regular dividers, and were designed to be a little tall in hopes of reducing or preventing migration over the top.  Box lids, though, are not very flat and may bubble outward making this more difficult.  It is recommended to place smaller, thinner items lower in the stack with other trays on top.  Placing a thin spacer under the stack may also help.

File numbering system examples:
FL3700-2w-third-2
Flambeau, 3700 size, 2 compartments wide, 1/3 high, 2 storage spaces

generic_open_4w
Generic, open side divider, 4 compartments wide

Other Notes

Material PETG
Layer Height 0.24mm
Perimeters 4
Infill 15%

Flambeau 3700
The Flambeau box pictured uses a combination of 1/3 high and 1/2 high trays.  These include single, double, and quad storage spaces.  The red trays are color coded for used hooks which are to be checked, sharpened and reused, or discarded later.  This prevents used hooks from getting mixed in with new ones.  Tray color coding may be useful in other ways as well.

Ozark Trail 3700
This is the nine divider Ozark Trail box.  There is also a five divider version for which these trays are not a good fit.
The box pictured uses a combination of 1/3 high and 1/2 high trays.  These include single and double storage spaces.  Tray dividers are oriented long-wise to better fit long jig heads.

Plano 3650
There are six compartments in which the trays will fit in the 3650.  The large radii in the corners of two compartments prevent a fit.  Also, two divider features in four other compartments prevent a fit there as well.
The box pictured uses a combination of 1/2 high and full high trays.  These include double and quad storage spaces.

Generic Open Side Dividers
These dividers were designed to be a reasonable fit in 3700 size Flambeau, Ozark Trail, and Plano boxes.  The fit will not be optimal in all, but this approach helped reduce the scope of the project.
The design intent was to increase space while separating dissimilar color plastic baits to prevent undesired color bleed from one to another.