30 years ago we used milk crates and it was a real PIA then. And it still is. I can still remember when the plug end of the cord gets hung up through the bottom or sides :'( Nylon bucket bags were / are a step up. But it is still Bags and boxes with stuff piled on.
I like your set up, It makes sense, it's on wheels, uniform sizes, interchangeable, can be stacked etc etc. Not to mention the tool mount table top on the same unit. It rocks big time for on site work.
Some will spend untold wasted time digging and rummaging through the milk crate for that part or tool for years before they would spend a few bucks and a day's worth of labor to make units like these. Time is money ??? But I got a feeling that the guy working out of the milk crate would not give them back once used for a job or two.
About the weight - theres not much getting around that unless you own a plastic injection mold factory. But at least you can tweek the plywood thickness to some extent.
Corwin - I thought of MDO, medium density overlay as a means of reducing the weight by using a thinner ply like 3/8". But it is not any lighter in weight than other plys. It's the Light brown paper coated ply. Comes one sided or two sided.
MDO comes to mind because it is a exterior glue ply with a paper coating. Used mainly for signs and exterior applications. It is very tough stuff, don't let the "paper" coating fool ya.
It's the durability and stability of the MDO, like Baltic Birch with a coating and ext glue. It would be great for the inverted tool mount surface because of it's smooth surface. Few or no voids. We can get 1/2" and 3/4" two sided MDO here at our local Menards, They also have Baltic Birch. Wow we got it good - Baltic Birch and MDO at Menard's

. Menards is like Home Depot and Lowes.
But 3/8" two sided is a special order through local lumber yards.
It's just one more option if going with thinner stock for the box sides. 1/4" Baltic Birch even ?