Harbor Freight has cheap casters - you don't want swivles, only rigid. Bigger wheels makes it easier to go over rocks and such.

I show using a 2x6 here - mainly because it makes it easier to attach the casters with lag screws. A 4" caster just barely - and I mean barely - fits on a 2x4, and I don't recommend it.

In use, you move the PDR around like a wheelbarrow. I use this to load my PDR on top of my Subaru. I wheel up to the back of the car, put the arms on the roof rack, go around back, and push it up. I then strap the whole thing to the roof rack.