We use compartment tiles in our fire trucks to keep equipment from sliding around. Compartments tend to collect dirt and tiles keep the dirt away from the equipment. I knew I would put some in the drom box.
I bought a box of 50 DriDeck tiles at Heimanfireequipment.com as they had the best price. There is another similar product called Turtle Tile. It is thicker and more expensive.
The tiles interlock and are easy to trim.
It only took about 30 minutes to do the top two compartments. I'll do the bottom after the box is mounted so I can allow space for the U-bolts.