Personally, I would start with the cylinders. Delete all but the top faces. Then cut the center one into quarters and just build 1/4 of the caster base by combining the quarter face and the whole face and then use a combination of bridge and append to polygon and extrude and target weld verts to fill in the gap and go from there... From there it is just a couple extrudes to get to here ... Did you model, light, and texture the entire caster? I was clicking through the photos you posted and saw a completed caster.