A set of railroad train trucks from the spares box.
Flip over a Roco Panzer III and open up the holes in the bottom of the hull and cut off the inner wheel retention barrier.
Remove the wheels and axles from the trucks and cut off the retaining pin on the outside of each wheel.
Cut the storage bin of the rear of the Panzer III turret.
Put the wheels in the lower hull of the Panzer III.