id used the ebay ones for 129 quid delivered
first job unbolt the 4 seat star bolts by sliding the seat right foward then back.
next measure the seat base left.
i made a 4 mm alloy plate for the top and used the orginal mounts to hold it in
then make the runner out for the new seats center and middle of the plate is a must.
drill the holes out .
then use m8 25mm bolts with locking thread to hold them in .
may be a good idea to test fit seat not bolted up first.
roughly 3 hours each one.
will add more pic in a few day.