if i were you, buy all the tubs you need first, or just one or two until you build the rack, whatever is best

measure the top of the tub, and say the tub is 35 in. long, i would make the shelf 36 or 36.5 in. long, and say its 18 in. wide, allow at least half an inch on each side of the tub so 19 in. wide


(these are just measurements for you to get the idea, i dont know the measurements of the tubs )