Go to your nearest Lowes or Home depot. In the specialty fastener section of hardware you'll find some threaded fasteners.They look like small barrels and are a very course thread on the outside and the indicated thread on the inside. Get the 1/4 X 20's. They are about an inch long. Drill a quarter inch hole, tap the fastener, to start it, then use a 1/4 inch hex wrench to thread the fastener into the wood. With a 1/4X20 bolt, clamp your seat down.
I know they work. I have used then to hold the wheel chock in place for my 870 pound Harley Ultra. I doubt your seat will need anywhere near that much strength.