I'm going through this currently as I'm building my tire carrier right now. I'm still humming and hawing...
Here's what I've got:
I'm going with delrin bushings instead of axle bearings. I've machined two top hat bushings that will get pushed in to the arm, leaving vertical bearing surfaces on the top and bottom. I've got a 1" dia cold rolled steel rod that will get welded through the bumper top and bottom. Then a steel spacer ring will get welded over the rod to provide a nice bearing surface for the delrin in the arm. I've cut a chamfer on the bottom ID of the ring to allow clearance for the weld bead around the rod.
At the top of the rod, I've drilled and tapped a 1/2" UNC hole, to which I was going to use a simple bolt and large washer to retain the arm. But... I've been thinking more about this, and maybe I should make this double-shear. I will have a structure welded to the bumper beside the arm anyway, as I need to replace the OEM plastic corner finisher which I won't be using. It wouldn't be much of a stretch to make this suitable for supporting a top plate.
The issues I have at this point... if I was going to go double shear, I wish I'd made the bearing section in the arm much shorter. I left it long for lots of bearing area, and then welded that diagonal in. If I'd done it shorter, I'd need much less of a vertical stand-tube on the bumper for the double-shear plate. That horse had left the barn already. I could still do double-shear, but the whole thing will look a bit clunky now.
On the latch side, I dunno yet. I was starting to think about using a paddle-latch as used on a truck/trailer. I'm wondering if I had a striker plate on the bumper to vertically support end of the arm, and an adjustable stopper so that the gate must be slammed to latch, and then the stopper will push the gate against the latch.
Either that, or what about an automotive latch? It could support the end of the arm both vertically, and keep it from opening.