Looks like 1/8" acrylic? Here is a tip to get you started.
For a simple shape where you are plugging a hole, cut the inner piece +.007 - .008 inches bigger.
For a more complex shape where you may have multiple inset pieces touching, adjust each piece by half that amount.
You will need a function like Illustrator’s “offset path.” There is something like it in Inkscape too.
See: Calling kerf compensation wizards: working with multiple cuts in contact with one another?