Forge, I have (newer model slab post-mount backrest) Ultimate Big Boy and Lowboy seats and an Ultimate backrest for them. It's OK, but it does not fit my low back because it cannot be moved down flush with the raised back end of the Ultimate seat like I need for my back. (many pictures on line show their backrest down flush the the seat back, but mine has a big space between them, and cannot be lowered without a cut and weld on the hardware; and you can't just cut off the bottom of the mount to lower it.)
Before I rode Ultimates, I pretty much always rode Mustangs, and I happened to have a Mustang Wide Vintage backrest from a Kawasaki 1500 with a post mount, and the post size is identical to the Ultimate. It slides right down in the Ultimate slot like it was made for it. It is also firmer than the (more spongy) Ultimate, but not hard (and can be adjusted lower and a bit farther forward than the Ultimate). I move it back and forth between my 2 bikes when riding them. I've been using it for years and been with other Valk riders many times, and no one has ever even noticed it.
I visited Mustang's site and cannot find the exact model for the bike I had as it is out of date. But I believe all their Metric bike Wide Vintage (or Touring) seats take the same flat slab mount backrest I have, and any of them should fit the Ultimate slot perfectly. (I cannot speak to their Harley backrests, assume they do not fit, esp if they have a tripod mount.)
I could not find a listing and price for the backrest alone on their site (without getting it as part of a seat package), but I am pretty certain you can buy a stand alone backrest from them (Wide Vintage/Touring metric backrest). I don't know what it costs (you may have to call them), and it may not be much different than the Ultimate, but the reason I explain all this is because there are probably some used ones for sale on Ebay or bike salvage sites like Pinwall. There will be a much better chance of finding a (correct) decent used Mustang backrest than an Ultimate.
Like here:
https://www.ebay.com/b/Mustang-Driver-Backrest-Other-Motorcycle-Seating-Parts/178033/bn_35090819If you happen to ride with or know anyone with a metric Mustang backrest seat (or go to a local bike night), you should be able to swap it right into the slot in your Ultimate (but ask first).
Other than that, I'd go with a Utopia, which I've only heard great things about.
Mustang metric backrest.
https://www.mustangseats.com/
Ultimate backrest (lowboy and bigboy)
PS; I had an Ultimate backrest for each of my seats. After I discovered I liked the Mustang better, I gave one of them to a friend in need. I have one left, but want to hold on to it in case someone walks away with my Mustang backrest. Neither lock down from underneath (like Corbin backrests do).
