I only have one photo, and it's not the best (taken while the paint was drying)
anyway, here it is.


I forget the size of the Gal plate, but there was one in bunnings that was about right, I think 380mm by 150mm sounds familiar?
but the total cost was:
1kg fire extingusher with CAMS approved bracket $35 from Super Cheap Auto
rectangle nailing plate $1.60 from Bunnings
satin black paint $2 (I had a can lying around anyway)
random screws for attaching extinguisher bracket to the plate $free (found them on the bench)
the bolt holes for the seat rails are 10mm diameter, you need a 14mm spanner/socket to remove them.
just remember to measure twice, cut once (or drill once)
also be aware that you need to allow for the extinguisher, mine sits a little too close to the seat and the ext touches the corner of teh seat rail, no problem, but it scratched some paint off the extinguisher.