I’d double that. The lower priced ballheads tend to get exaggerated in their load ratings. Also, your box will extend several inches in all directions from the head. Which will increase the the downward pressure. You’ll probably be fighting creep with a light weight head.

I have been trying to do what you guys suggest to no avail. I’m finding it VERY hard to get specs on weight load on most sites, including eBay and Amazon. That’s even assuming they are accurate. Yes, you are right about the pressure of leverage. I’m actually shooting for something rated reliably at 15 lb. or greater, those 8kg. ones. It is not easy to find one in the price range.
Also, the scourge of online reviews is that you never find one thing that doesn’t have at least some negative reviews along with the laudatory ones. So it’s a hard slog.
If you have a specific one you’ve found to point me to, please do!

Folks, I appreciate your efforts, but having pursued those avenues yet again I find that they are far too expensive for my tastes.

Luckily, I was able to figure out how to make my lower end tripod head removable, and it appears that the leg assembly may fit one of the not overly expensive heavier duty pan heads on Amazon. So I’ve ordered that to see if I can get an improvement within my budget. There was never anything wrong with the leg assembly, just that my 10+ lb. kit was just a little too heavy for the pan head to hold rock steady under pressure sometimes, for which it wasn’t really designed.

Thanks for trying, though.

The thread is not wasted. There is a proliferation of the kind of pochade setups I use (box on tripod) and many of them report the same kind of issues with their OEM tripods too. So this is a use for field tripods that is lacking enough market clout as yet to have sturdier and not ridiculously expensive solutions. Most tripods are meant for cameras, and I see from Web research that those with even heavy lenses have this same issue, which is magnified for photographers.

Whoa! May have solved my issue quickly and inexpensively after all.

I got a BSR International heavy duty pan head on Amazon for $23. It fits well on my Dolica ST600p tripod, once I figured out how to swap heads.

Appears to be very strong, at least testing it out in the house with my 10+lb. Guerilla Painter 9×12 box fully loaded. Best of all, it appears that my Dolica quick release plates might work on this after all (Arca-Swiss standard?), so I don’t have to buy extra plates. This thing is rated at 12kg., which is about 26lb. so pretty heavy duty for my uses.

We’ll see how this holds up in the field.

Thanks for everyone’s help.