Update on this having just been through it.
First: The Dorman #13973 readily available at the corner parts store does not fit. It's too big and stout enough that manipulating it doesn't work. It's the closest thing they make per corresponding directly with Dorman.
I searched for the Avonride trailer dust cap from the UK that Rob Siegel used (Pick with the "A"). Yes, it's readily available if you can find someone who ships it to the USA. None of the ebay options or vendors I found would ship it here.
Having read that one from an old Ford Cortina or Escort would fit I searched that. You would think "Ford", I'd be able to find one in the US and you would be wrong. I found a vendor in Germany of all places. They shipped it as an uninsured letter without a tracking number and it took 15 days to get here with a $16 +/- price for 2 dust caps and shipping. Worked great!
Other options that I did not try were a small trailer vendor or NAPA. I was waiting on those two if the caps from Germany did not work.