my take on the PS Technik Skater Dolly
Wheels are Skateboard wheels
I used stainless steel rods for the wheel support and also for the camera platform support with ball bearings on it
(same size as the skateboard wheels bearings)

After a couple of uses i recognized the clearance is quite small, so i suggest the wheels should protrude further out of the housing.

this is by no means a final design, more like a working prototype, with room for improvement.