Some style of springs are called a slipper spring.They use them in big truck apps. Bolt only in 1 end. Other end rides on frame.I have them on my Crawler trailer.
I have Tempress also but mine are different. Love them.
This is what I have.
https://www.ebay.com/itm/Tempress-54901-NaviStyle-High-Back-Boat-Seat-Blue-Gray-Marine-Hi-Back-/281761284332?hash=item419a4800ec:g:miwAAOSwDk5TvxeT&vxp=mtr