Do you have a pic or two of your setup?
I just took out the Pants Savers yesterday to give them a thorough cleaning and removed the plastic sheet in the process.
My Pants Savers fit my floor pan quite well. All you do is use a tape measure to measure the width and length of your Pants Savers and add 12 inches to each dimension so that you get 6 inches extra on each side that rides up the sides of the carpet area. Of course if you have a flat bottom floor as some cars may have, that won't work.
The tarp clips are slipped over and tightened at each end of the rear of the Pants Saver and ties to the seat rails. This prevents the Pants Savers of sliding forward.
My neighbor will be doing that to his next car after seeing how well my set-up works and how well it preserved the interior. I do likewise for the passenger side. As for the rear seats, I do the same but skip the tarp clips. It doesn't look great, but then neither does the car during the winter months.
Floor pans don't rust from the outside in...well they do, but they rust from the inside out much quicker.