Bigjaype
I'll eat Sonny's sandwich...
I bought a universal backrest and discarded the seat/fender mounting hardware.
I bought some 50x50 aluminium solid square and had it machined as per the pics.
I no longer have the machinery to do it myself so it cost. But I had enough material for two. The second one is up for grabs. I have the drawing if anyone is interested in machining their own.
Two of the holes I made M4 as there wasn’t enough meat for M6...after matching the slot for the backrest to slide into.
Below the finished product and items I bought.
All in all, below $200...and if I can sell the other mount, I could buy some beers.
ALUMINIUM Square Bar 40, 50 or 65mm Various Lengths 6106T6 FREE POSTAGE | eBay
https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.com.au%2Fulk%2Fitm%2F172775341125