It might serve the purpose but looks like a toilet seat, sorry. LOLHow about this? I had actually posted about this a couple of weeks ago.
I have a 2012 AN400. I wanted to get a back rest for my wife, but as you have discovered, they are hard to find/expensive. We also wanted extra storage for when we go on an extended trip or just want to get more groceries than normal.
We ended up going with a Givi Monokey mounting plate and back rest. Total price for both, with shipping, was $178 from Competition Accessories. We haven't gotten the top case yet, but it doesn't look half bad without one.