I’m waiting on a question to be answered by Dayton Audio (one of the manufacturers of these) as to where I should place these. To me it makes sense to install them directly on the seat, but their website says to install them on the floorboard under the seat. Confused.
The ones I've seen were mounted to the seat firmly secure them to the springs with some chunky zip ties or baling wire or similar. Pretty sure they're also used in home theater mounted securely to the frame of your seats. I don't think the floorboard will do much for you.