I wanted to make a smooth-rolling dolly that could bring the camera low to the ground, so found some inline skate wheels, four for about $20; I found a set of eight axles for $5. Then I added a bearing to couple the two halves and the rest of the design is printed. Most tripod heads should fit, there is around 70mm of room between the thumbscrews.

The steering angle can be altered so you have some control over the curve the camera travels. When the steering angle is set two M5 bolts are tightened using the supplied thumbscrews. 16mm length bolts fit well but 20mm or 25mm would also work. 

The central bearing is 35mm ID, 47mm OD and 7mm high. Found it at Amazon. Kind or quality are not particularly important as long as it is the right size. If you don't want to use a bearing you could just print a filler piece of plastic to substitute for the bearing, this will not affect the wheels only the steering ability. Or even leave it out entirely - with the central camera bolt as pivot the two thumbscrews will hold everything in place just fine.

Printing is not really critical. The top half of the dolly in the pictures is printed at .15 height with the bed vertical, lots of supports, to ensure no adhesion problems on the narrower parts (wheel arms) and 100% infill. The other half is printed at .3 height, flat on the bed with 15% infill and no supports - it seems just fine for light cameras but for a heavier camera and lens the longer print is probably a better idea. (Of course the top half looks a lot nicer too).

If you want it to be slightly higher, just turn it over, it rolls just as well either way - small ball heads or other mounts will still fit inside the bearing recess allowing the usual angle adjustment.