There are some options available but I can only give ideas because I went the expensive Mustang seat route.
When I first got my bike I too needed a backrest. At the time I purchased a Kuryakyn Tombstone bag. It sat on the pass seat and strapped to the bike. It had an internal framework and pressed against the pass back rest. It had several different thickness pads that zipped to the front for length adjustment. They are discontinued but maybe there are some used for sale or something similar.
My wife has an Aero with a two piece seat. She found a backrest, on Amazon I think, that goes between the two seats and attaches to the bottom of the drivers seat. It works and she is happy with it.
Lastly, the more McGiver option. Get a medium gear type bag and pack it tightly with old heavy clothes and strap it to the pass seat. At least it would help till you find something better.
Good luck.
Eric
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