Step 2: Constructing the plastic insert
If you're using relatively thin plastic board material, you can probably use a set of heavy duty scissors or tin snips to cut out the insert.
The dimensions (in inches) are shown in the drawing above and are approximate. You might need to do some sanding for a better fit.
After you construct the insert, try sliding it into the bottom of the enclosure and putting the top on. If it doesn't fit properly, get the sand paper out and sand down the parts that are interfering.