Hi,
I have 2 extinguishers fitted

, one to the roll cage bar between the two seats, and one in front of the passenger side seat.
My mechanics fabricated a small bracket that bolts onto the front of the runners of the seat. Two bars run forwards from the runners. There is a cross bar horizontally in front of the seat, joining these two bars. The extinguisher and its mounting bracket attach to this
There are some photos attached, but it is hard to get perspective
In the photos of my car I obviously have race seats, but the priniciple is the same for the standard seats.
A couple of hints,
Be careful where you locate it, as it can restrict seat forwards motion. For this reason, if you only need one extinguisher it is best to put it on the passengers side
Also, make sure that it clears the lever that adjusts the seat.
GoGadget has similar brackets with standard seats
Cheers
Robbo