The cheaper Sterilite 6qt shoebox tubs have changed both size and shape over the years. I had to use the slightly more expensive Sterilite #1751 clip-top shoebox tub to get 4 across (see my photo a few posts back). They work just as well and accommodate 4 across with a bit of space to spare. They're still 6qt but don't have the taper that the standard shoeboxes have, which makes them ever-so-slightly too wide by an inch of so when placed 4 across.
I bought my #1751 tubs at Target. I believe that the tubs that Justin Kobylka used in his original design were Iris tubs that I think can be bought at the Container Store, but I don't see them on their website. You may want to drop Justin and email and ask him yourself, but chances are Iris tubs will cost more than the Sterilite #1751 tubs that I used. I think I paid $2 each when they were on sale.
I hope this helps and doesn't discourage you from using this rack design. I've been using mine for almost a year now and I love it! I almost wish I had a reason to build another now, just in case Lowes changes the design of the shelving units, if they haven't done so already. Good luck!![]()