One other tip is to rout out a circle in the wooden rails big enough to take a nut. Then you can bolt the seat rails firmly to the battens, giving a seat with wood rails firmly attached and the 4 now captive bolts firmly in position.
You need to to rout out a hole to allow the nut to not stand proud of the wooden rail, and it needs to be big enough to drop a ring spanner round it so you can do it up good and tight.
You can now drop the seat into position and the bolts will go through the mounting holes neatly. It makes removing and refitting the seats in future a complete doddle.
HTH
Tim
