Four 1/4-20 grade 8 bolts with heavy fender washers.
Bolts into a section of 1 1/2" square tube with 1/4-20 threaded inserts welded into one side.
Square tubes are welded to the factory seat channels on the floor.
On an aluminum seat, drill through the seat and mount.
With a composite seat, use the 90* angle side mounts.