The interlock probably isn't completely necessary. A few weeks ago I was researching building some tallie crates. Found this picture on the Coopers website. If I don't end up in Canberra for a couple of months over summer, knocking out a few similar to these will probably be my Christmas building project. Hardwood is expensive here in Cairns, so will probably just be ply.
As an aside, I like how they have full crates stacked on their sides in the background. I'm presuming it made them easier to grab hold of and lift without bending over; maybe one in each hand.