I made a set of machine skates a few year ago. One of the best things I done, as it's surprising just how handy they are to have.
Here's my castor one, although I've still never made a handle for it -

I used a big thrust bearing, which I machined a groove to help locate it on the base plate -

And welded a stub onto the rotating plate, which has a bolt/big washer on the underside to hold everything together-

And I've got two of these-

I just welded the axles in for simplicity. I did consider drilling/tapping them to accept retaining bolts, but if I ever do need to dismantle them, the weld can be ground of.
They were all built from 1/2" flat plate, as I happened to have a big sheet of it lying around at the time.