What is the exact diameter of the part that goes into the axle? (I would like to connect a Qerido Qubee to a Thule axle)

I converted a Leggero dog trailer to a Chariot/Thule hitch using the Kugelastomer. Worked perfectly for me. Ideally, you should buy the safety strap at the same time.

It works with Thule

The round (ball elastomer) did not fit into the square (drawbar). I had to print an adapter and now it works.

I have mounted it on a Leggero bike trailer and it has worked so far.

Yes, but only with the matching counterparts

Yes, it worked well for us. With manual dexterity and a drill :-)