Regarding the adjustable casters mentioned above, I took a slightly different approach.
My XL cart, currently under construction, is about 53 wide by 34 deep. It’s a stout 3/4" plywood box with a 3" thick top of laminated MDF. No torsion box. I did try 4 standard 3" casters from HD and discovered just how out of level our tract house garage floor is. The cart’s primary location is the only spare space in my shop, up against a wall, under a window. Rolling the cart anywhere else resulted in one wheel or the other lifting off the ground, which would no doubt lead to some sort of wracking or instability.
After toying with the idea of a 3-wheeled cart, I expanded on this with a design I first noticed while delivering appliances in my college days. Some washing machines used a pivoting beam across the rear of the unit, with the rear feet at the ends of a beam which pivoted at the rear center of the unit. The effect was a unit that automatically compensated for unlevel floors. The pivot point acts like a phantom “3rd wheel”.
So I built one. A 42" laminated plywood beam, pivoting on a piece of 1 1/2" galvanized pipe. Single castering wheels at each end proved a bit unstable as the castering action placed them off center, so I switched to a small metal plate at each end, mounting two smaller casters set as far apart as possible. Now I realize this doesn’t assure the cart top will remain bubble-level, but I figure the main concern is a top that wracks, twisting the spoil board. As long as the top and spoil board remain in the same plane we should be OK.
And it works. I can run any wheel up on a 1" thick wood chunk and the cart won’t wobble. Now as built, it was possible to force either end down at a funny angle with enough weight, so I added a rude damping system to the tops of the ends of the beam, by gluing on a small pyramid of foam made from a cut-up HF floor mat. They barely touch the underside of the cart, compressing increasingly as the beam moves. This greatly stabilized the cart but doesn’t detract from it’s ability to remain flat-footed on the wavy floor. With two locking casters in front, it seems like it should work just fine. I say “Should”, because I haven’t remounted the XL and cut anything yet. At the rate this cart is evolving, . . . and then there’s the enclosure to finalize . . . . Someday soon. Maybe.