Replacing rubber connection in a bicycle trailer coupling
I suggest undoing the bolts that hold the rubber piece in place.

Obtain an old car tire, and with heavy shears, cut a couple of pieces the same size and shape. The idea is to put them back to back to make up the width.

You'll need to make holes on each end for the bolts. A large drill could be used for this; I suggest a drill press, rather than a hand-held drill, for safety reasons (the drill bit may bind in the rubber).

You may need to have a few tries to get it all to work right.

Good luck :-)