Bolt through an inch thick (or less) piece of timber that is at least the size of the swivel. Sandwich a ratchet strap or two, endless preferably, betwixt timber and swivel. Ratchet around the seat board on the boat and fanny is your uncle
The timber here (just CLS) is overkill but I'm lanky and like a high seat. Glue a mouse mat or similar to the underside so you don't overly annoy the boat owner by scrathing their board (or at least make sure you've countersunk the bolts). Enjoy!