Swivel seat mount ?

Swivl-Eze 238 Series Seat Mount, Forward Handle, 3-Degree Angle, Plated Steel 238153-1 - The Home Depot

Cheap Jon Boat Seat Mounts - review of clamp on swivel style seat

The SPRINGFIELD™ Square Swivel Boat Seat Mount with 0-degree tilt and 360-degree rotation is designed with a universal 4-hole mounting pattern to fit

Springfield Marine Non-Locking Swivel for Boat Seats - Four Hole Mounting Pattern - 6.75

Swivel for use with 360 Seats. Includes mounting hardware.

Swivel Seat Mount

Alpine Mechanisms Swivel Seat Bases for Transit - Agile Offroad

Garelick EEZ-IN Millenium Series Smooth Swivel Seat Mount

Springfield's 1614201-PP Seat Swivel is designed for use with KingPin Series Posts (sold separately). The swivel rotates a full 360 and fits 4.5 inch and 5.5 inch square bolt patterns. It complies with ABYC requirements for unintentional detachment. Patented design for optimal strength.

Springfield Kingpin 7 x 7 Seat Mount w-Spring - Polished

Shoreline Marine Seat Bracket Jon Boat with Swivel

Garelick 75018 Universal Easy Mount Boat Seat Swivel features larger bottom half to expose four mounting holes. Rotates 360 degrees and fits standard

Garelick Easy Mount Universal Boat Seat Swivel

Sprinter Swivel Seat Base Adapter with Offset Pivot – Sprinter Upgrades