Today I picked up my new mountain bike
A north shore 4 bike rack.
I noticed even with a threaded anti rattle hitch pin there is still vertical play between my hitch extension and receiver hitch.
I eye balled the gap and cut a piece of 2.5mm aluminum scrap I had into a shim.
I greased the heck out of it and the receiver and slid the extension in, it fit great in there with still some room to move not jammed in.
So now my extension (7inch) sits flat to the receiver.
I tried the same thing with bike racks 2 inch bar but there was not much slop in the connection , I would have needed 1 mm piece. Possibly some stainless sheet from hobby store would have worked.
Here's the rack extended.