I put 1/8" in all my racks. When I put shelves in, I put the tub on the previous shelf and put a piece of 1/8" wood on top then lay the next shelf on top of that
I just built a hatchling rack, i bought a peice of that masonite board that is 1/8 of an inch, cut it to fit the shelf and laid it on the bottom and put the tubs on top, then laid the next shelf down.