I built my own "heavy hauler" a while ago. Aluminum welding which was a major pain. I need to practice more as I think I'm figuring out why I was having so many issues (trying to run a bead rather than a string of puddles).
The original plan for the trailer was a bi-layer frame with an upper section that would be flat, and large stuff could overhang the wheels.
Frankly, I use the cargo bike more as a tow machine than for hauling, so my next incarnation will be specialized just for towing.
I do prefer a more typical kid's trailer for more compact stuff, and have one that I've gone "skeletal" with. I had intended to strip and rebuild, but it turned out that the fabric floor and sides of the kid's trailer works well for most of my hauling. But, for some bulky things, if it fits on the kid's trailer frame, I'll just take a bare frame.
Another plan was to make a telescoping frame using aluminum tubing that would slide into or over the tubing for the kid's trailer, but with the "heavy hauler", it hasn't been a high priority.