I use a SitGo. It's a little heavy at 1.7 lbs, but is perfect for me as it doesn't take up much room and helps with a better perspective than regular sitting height. I've used it in different countries and it's seen the cobblestone streets of Havana and sitting on the side of a mountain in Austria. (I've even pulled it out while waiting in long queues at the airport.) Is it for everyone? No. It does require balancing as it's making me a tripod.
I think the interface between the quick release plate and the tripod head is a prime spot for wiggling. Is that where you're having problems?
To solve that I prefer an Arca-Swiss style connection because of the large contact area and clamping action. Mine (like all the cheap ones) are generally closed by screwing the clamp closed, so they're a bit slower to use than the spring-loaded or lever actuated options, but they make a very solid, stable connection. Between that and making sure your QR plate is firmly screwed to your box, you shouldn't have any wiggling between the box and head.
My favorite ballhead is this cheap one: https://www.amazon.com/dp/B07RXQTL1V/ . It would be kind of annoying for photography, but is great for plein air. It's solid enough to handle a large pochade box, and all the movement is controlled by a single knob, which is lever-shaped and easy to lock down.
Since you mention lightweight, the Marsace XT-15 tripod is also a favorite of mine. It's tall enough for me (just under 6 feet tall) to use standing without extending the center column, very compact when closed, super light (2lbs!), lighting fast for setup and take down thanks to its internal leg locks, and has a head well enough suited to plein air that it doesn't need to be replaced.
The backpack is only $10! It’s big. I cut out the inside laptop holder so the box slides in and out smoothly. The top rolls down so there is plenty of room for extras. Now the pack will not have room for your giant tripod once packed with your giant box so you’ll buy some straps for $8. Slide one leg into the outer pocket and then tighten around entire pack. Will also hold a clip on umbrella on the other side. From pocket for dry panels or snacks or flat water bottle.
Linkbackpack
I use this. It's worked pretty well for me and held up to some wind. It's longer on one side than the other which has actually been beneficial so I can orient it to protect my canvas and palette but not block my view.
4 Piece Metal Rectangular Tin Metal Hinged Lid Tin Metal Empty Box Container Silver Rectangular Storage Tin Box with Lid for Watercolor Jewelry Makeup Pill Candy Craft Organizer (7 x 4.25 x 0.59 Inch) https://www.amazon.com/dp/B08YYV5RLD/ref=cm_sw_r_cp_api_glt_fabc_51WF8T5GFF48KEH1A7QM?_encoding=UTF8&psc=1
Gitzo makes a model that is just like mine, one pound heavier . Mine is ten years old, worth the money.