If I design one, I'll make to fit onto a $15 Harbor Freight furniture dolly. Cheaper than buying caster wheels, and can be set in a truck bed without the wheels for stability.
What I use for an engine dolly on old engines (usually short blocks) are the Harbor Freight car dollies.
They don't roll very well with a car on them, but do just fine with an engine.
They could have brackets welded to them to bolt an engine on them like AndyF's pic.
They would be strong with less fabrication needed than building from scratch, you just need brackets to bolt the engine in place.
