Prefer the seat mount one... cheers.

 

My car's prior owner made a simple one by bending a steel strap in a loop fastened by two screws on each side to the seat's sliding bracket extending just past the seat adjusters and riveting two quick-release ski boot buckles to the loop to hold the canister. A ski shop should be able to supply the buckles taken from a discarded boot. A small 1/2" x 1" square of high density foam was glued on the loop between the loop and the middle of the extinguisher. The 5 lb extinguisher now securely sits about 1/4" off the floor at the end of the seat's bottom cushion. Filling that gap between the extinguisher and floor to stop any vibration is another 1/2" x 1" piece of foam.