Here's the problem.
You have some of those organizer bins that have lots of drawers and you lost the plastic dividers that go in the slots.
You also have some of those slot covers that are used in computers to cover the slots where the option cards go.
This is the drawer
Measure the width with calipers.
Mark the distance using a metal scribe
Cut along the line you marked
Flatten the metal strip using a hammer.
Cut off the end.
Notice, you have a curve. This has to be flattened to go in the slot
Use a vise to flatten the end.
File the sharp corners. This will help with insertion and also prevent cuts from sharp metal.
File the end. The end is sharp and can not only cut, but it won't slide easily into the slot otherwise.
Inspect your work.
Test the fit.
Voila!!
It takes about 3 minutes to make one slot.