about 15 years ago i got my plastic padded flip down seats at wal mart (2 of them) - then got the clip on swivel base to screw on the seats -- i had BAD LUCK with clip ons THEY JUST ARE A PAIN in the ass and always slip and the nuts that they come with are the pits!
HOWEVER - this worked great for me - take the clips out and get LONG eye bolts that fit the clip slots (4 for each seat) adjust the eyes to extend out around 3 to 4 inches (or what ever works for the boat bench seat ) and tighten them up!
now get your self 2 rachet straps per seat - attach hooks to both the back eyes and bring straps under the seat to the front eyes and attach hooks and tighten (don't tighten to hard - i have a fat ass and i just snug mine and haven't fell overboard yet)-- the seat will not slide and rotates with ease!! and the seats are easy on and off! you will know how much of the strap to cut off and heat cut edges to keep the strap tips from getting fuzzy!
they were a lot cheaper 15 years ago! BUT I WILL TELL YOU THIS when i got mine and put them all together it seemed like a fortune to me! (THAT BEING SAID) all it is going to take is a couple of longs days and rough weather in the boat and you and your back and neck will be loving the investment! and you and your fishing partners will think it was worth every penny!!!