I line all my truck tool boxes with that horse mat from tractor supply. A quality utility knife will get about 3/4 through it, bend it back on itself along the cut and its easy to finish it off with the knife and keep a clean cut.
Im gonna order up the interior mat for sure, but may make my own for the bed.
tractor supply also sells the rubber mat by the foot in thinner selections iirc I saw 1/4 and 3/8 it was 4' wide and $5.50 and $7/ft respectively so it would cost between 16 and $21 if you went that route. I like the thought of the 1/4" for the bed because you should be able to roll it up and stick it behind the seat If needed. I may go get the 1/4 tom and cut me one for the bed.