I have a few drill bits, screwdriver bits with magnetic holder etc stored inside the case with my battery drill - a DCD796D2. However, whenever I close the case and move it around the "bits" and pieces would scatter around the inside of the case. So I designed a box to house the smaller items that fits in the centre compartment and stays in position when the case is closed.
This was printed in PLA and I am not sure how durable it will prove to be, so only time will tell. At the time of publishing this, the box has been in use for 11 months and has not been cracked or broken. PETG might be better material though.