Is your Redseal one with a floppy floor or slats or wooden floorboards and no keel? Is the aim for your comfort or to tighten floor|?
Those gym airmats come in a standard rectangle so not sure how you'd trim it to fit the boat? If they were available small enough to sit in the floor one would obviously spread the load and add to comfort but would do little to improve performance unless it was a perfect tight fit and you added a sausage keel between mat and floor.