can you do some kind of drawing. I have some nylon roller offcuts that might be able to get to the sizes you need and a lathe, and us mad landyachter love a challenge

Rather than have a lame attempt at drawing I'll show you.
I do have the supplied inserts which do fit but have flanges on the end. the tow bar attachment will not fit if these flanges are there.
At worst I will remove these flanges and use the remaining "tubes" but I would rather manufacture a tube in one piece to do the job. and if I make two I will have spare.
As stated before, the internal and external dimensions are rough due to the lack of precision measuring equipment, but they roughly are:-
Length - 25mm
inner diameter- 7 or 8mm
outer diameter - 12 or 13mm

First photo-you can see the bike hitch with the hole and the quick release that fits inside it.


2nd photo- this is how the two will be when in use, just needs the insert



3rd/4th photo- this shows the quick release with the supplied inserts






5th photo - an insert with a ruler for size perspective

can you show what it looks like when you attempt to assemble it, so i can see why and what to change?
it looks like it should fit.