This filing container is built specifically for holding suspended files. In one direction, it is suitable for letter-size file pockets. In the cross-wise direction, it is suitable for legal-size file folder pockets. The smooth second rail molded inside the edges on both sides of the file container allows the suspended pockets to slide forward and backward smoothly. Clear tabs hook into the edge of the pockets for labels.
If you purchase the correct file pockets for the container, you would not need the plywood divider as each file pocket is its own divider. Several folders can be placed in one pocket if desired.
That said, your requested modification is rather simple in theory
As you wish to re-purpose and modify the smooth continuous sliding folders within the container for functioning in steps of 1½ inches, begin by notching the rails in intervals of 1½ inches. This kind of plastic is brittle so care must be taken to avoid shattering and splitting the plastic while you notch it. Grinding may be the best non-destructive way to do this. You'll find the best way.
Notches on the rails should be placed accurately so the plywood divider will hang square and perpendicular to the sides of the storage container.
The plywood plug will need something to fit into the notches so drive a nail on either side partly into the edge of the divider. If the divider is allowed to hang by the nails a fraction of an inch above the bottom of the storage container, its weight will help to stabilize the divider to hang upright.
Here's an idealized front and side cut-away view of the divider with a nail placed for hanging from a notch in the rail to prevent sliding inadvertently.