Those low swivels are nearly impossible. You have enough room to get a nut on it and that's about it. I used 1/4"x20 flat head ss bolts with locking type nuts and screwed it to plywood from underneath. Also used three pieces of 3/4" plywood about 6x6 inch to keep it from scraping.
I am looking at a swivel seat. But I have been looking at several different places to mount it. One is place the mount in front of the bench seat on the plywood but I worried about it being sturdy enough or mount the plate to the plywood which is attached to the bench seat.