One of the makers of pvc racks used to use a piece of perhaps lexan dowel rod right in the middle of each level for support on racks using 2 tubs on each level. It would be quite easy and inexpensive to use dowel rods made of wood and painted white with your melamine rack. This is just from a lazy mans perspective. It seems to me that it would solve your problem as well as a full board would with less work involved.