A big tire - say, 31 x 10.5 pickup size - and a few tie-downs will work OK.
What I did for my BBs was to take some 2x8 and 2x4, about 24" of each, and screw them together using Simpson ties, all from Home Depot.
Those ties are @ inside & outside corners.
The width & length match the oil pan rail, the 2x8 along the sides, and the 2x4s ft & back across the bottom so there's clearance for the harmonic balancer & rear of block.
Lumber oriented vertically, of course
It basically fits around the pan, with no worry about proper support, rocking, or falling over.
Absolutely bulletproof, will outlive me, and easy to add a furniture dolly or dedicated wheels later if needed.