I get it.
I have been looking at some of this type of thing.
Tractor Supply has some that you can kinda build your own and I was looking at doing it with a trailer spindle shaft.
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Otherwise, you COULD use a spindle from the rear of a front wheel drive car and grind/torch/cut off the outer part of the hub to get it down to a size you need. The bearings would be easily obtainable and you would know where to get seals. It would have dust cap/cover on it. Could also do one from an implement dealer or an old wagon or something from a piece of farm machinery.
The main thing is making the spindle go far enough DOWN into the bumper that it can pick up the bottom of the bumper to support the overhung load. You can do this with a piece of pipe or shaft out the back of the spindle.
Good Luck!
I kinda though this was what you were asking, but wanted to make sure.