I modified the idea from an IDPA article.
They are simply a slightly offset 2x4 "T" supporting 3 equidistant 1 1/2" pvc pieces 16 1/2"apart to hold the furring strips. This holds a standard IPSC or IDOA cardboard. The wheels are 10" pneumatics (Harbor Freight, $4.99). The axles are a 3" section of 5/8" thread rod welded to a piece of 1 1/4 angle iron. A 3' piece of rope on the back (the tail) makes he easier to move to reset them.
We move them using ropes and pulleys attached to dog stakes. They roll quite easily over fairly rough ground. One person can pull all 5 of them at once if desired, from a safe location. They were moved individually, in a train of 2-5 or multiple targets in multiple directions.
They were very stable, rolled straight and were untipped by moderate wind. Positioning the posts in the different pvc pipes allowed for targets moving straight towards or away from the shooter or crossing shots.
We used a dedicated target puller or clipped the drag lines to the shooter so the targets moved when the shooter did.
Yes. I know my welds look like a hippo's hemorrhoids.

My shootin' buddy got these wheels for free. At the same time he also got about sixty 12v motors with gear reduction boxes on them. So yes, we thought long and hard about motorizing these.
After playing around with these RATS we pretty much figured with the variety of scenarios we wanted to run it wouldn't be worth the expense (batteries, switches, relays, spools, extra gearing, etc.), the labor and design and the loss of flexibility for a good variety of set ups.
With 5 stakes in the ground, caribiners on the pullies we can run numerous setups. We also have the "human" factor. Random order of multiple targets, uneven speeds that throw off your aim (deliberate and accidental) make it more challenging.
eg, All five targets might start at once or 2 may move to be engaged then the other 3 may charge you....