Jo, I made a rack like that out of 3/4" melamine coated particle board and it was only a 2-across(the 32qt tubs), and it bowed, making the tubs hard to slide in and out. If you're using those 32qt tubs, I recommend making a rack that is just single width and just go straight up with it, maybe 8 tubs high?

If you want to lighten them up a bit, using 2x4s for the sides instead of melamine will help substantially. I would still use it for the shelves though since it's waterproof. Home Depot also sells this water-resistant hardboard that is $4 per HUGE sheet, and that would be great to use for the sides(just nail them on with finishing nails in the spaces between the 2x4s).

This is what I am planning on doing just to lighten the racks up and so they are much more sturdy. Melamine isn't very sturdy and is easily stripped. It also is incredibly heavy so it tends to bow if not supported.

I have 2 more baby racks to build as well as another 27/32qt rack(can't decide on tub size) and another 41qt rack, so I'll post some drawings of how I'm doing mine. Maybe that will help you with some ideas