I just built one , to get ideas on a latching mechanism , I looked at a number of manufacturers web sites , most have a gallery of pics . Go look at Modern or National , if they have a bumper in stock / on display , look at the latch .
I used to build trailers to pull behind snowmobiles , the latch design on those was a scaled down version of what many bumper/tire carrier use . Basically a T handle with a captive spring , inside a tube , a washer is welded to the shank of the T handle , the spring provides down force against the washer.
The hinge pin , I made that too , I have a 12" x 36" lathe , settled on a 1" fine thread , bushings ? , motorcycle fork bushings or other similar bushings would be sufficient .