Hello Daniel,
Apologies if this isn’t going to be practical - always difficult to gauge from photos- but can’t you sleeve a suitable small dia. brass/ hard plastic etc. pipe down into the section in the tailgate, leaving enough to push the plastic piece over.
Obviously you would need to find a suitable dia. sort stub of pipe, or get one turned down. Thoroughly clean both pieces and Araldite the sleeve pipe in.
Easiest way to measure the OD you will need is to carefully insert a drill bit etc. until you find one which is a snug fit.
Kind regards,
Gareth.