Dave asked a question in a pm and I was trying to respond there but couldn't figure out how to attach pictures to a pm so I am posting here with some additional details on my version.
All made from 3/4 plywood.
I took a couple more pictures that might help. I built it a long time ago and don't remember the exact construction method. I didn't really want to unload the whole thing and flip it over but I will try to explain what I can see.
It looks like cut a bunch of strips about 4 inches wide. Then I built a square for the base. Then added a taller strip on the long edges to act as a stop. It also looks like I may have doubled up the ends of the box since they are supporting the vertical supports. So when you get done there is a double thickness all the way around. I think there are two cross pieces at 1/3 and 2/3 the length that are used to mount the verticals (guessing they are double thickness also but not sure).
There are four vertical supports that are built at a slight angle with bracing at the very bottom, very top, and about halfway in between. These are then screwed through the bottom brace into the short ends of the base and into the two extra cross pieces at 1/3 and 2/3 the length.
Then there is a shelf added at the bottom that goes inside the vertical supports at the bottom and two that go on the outside of the vertical supports between the vertical support and the taller strip acting as a stop. There is also a shelf that goes inside the vertical support at the brace halfway up. This is very important because it provides rigidity to the verticals so when you grab them to move the cart around the verticals don't snap off. I didn't do it but you could also add a "shelf" along the very top to tie the verticals together one more time. I don't think it is necessary but would make it built like a tank if you did. The cross braces supporting the shelves are about 3" wide.
It feels like I just added more blocking under the corners to mount the casters to.
