[BigyellowFJ]

I used the stock mounting hardware that came with the extiguisher. I removed the cap on the arm rest and that gave me access to the inside of the door without having to remove the door panel. Then I used stainless steel bolts and a good sized flat washer on the inside finished with a nylock nut. This has proved to date, a secure and rattle free install. I have taken the truck off road often and no rattles and no flying extinguihers

As for seat clearence issues, none from the front seat, you have to sit in the back seat with the extingusher and have someone in the front moving the seat back and lean back in normal driving positions. Then mark where you need to mount it. Remember that I used 2 smaller sizes than you might first pick. Thats why the marine application worked as boats have space issues too.
Now when you fold down the rear seat, you have to open the door or remove the extingusher temporaraly. Again, nothing is perfect but it works for us and I love the quick access.