iris all the way anything else is 3rd rate tubs!
they can also be found at bed bath and beyond the mcb-ss 5.4qt its a great hatchling tub for 1.99.
thecontainerstore.com or the store carries their own sizes of iris tubs which are smooth inside as well and can be ordered in bulk for slightly lower prices. the shoebox tub will work in a mcb-ss rack same height roughly but the are small in width and length.

the mens boot box (iris 375)is about 15 quarts and they are also flat.