You can make on quite easily.
A lenght of st.st stud iron, 8mm/10mm thread.
Bend in vice the square shape (for my trailer box section), ensuring it will pass through the wheel holes.
Make a holding plate, rectangular with two 12mm holes, to fit the bent shape.
Then assemble -threaded bar around the trailer box section,then put on spare wheel ensuring threaded bar goes through two holes, fit on plate and tighten two nuts and washers (Nyloc nuts).
Watch where on trailer you fit it, front/back (it will go in the water every time you put your trailer in), and what orientation as you don't want it to collect rainwater.
All bits from B&Q for a couple of quid.
Worked for me.