I used the 1/8 inch thick black pyramid design rubber matting from Restoration Specialties and Supply Inc. I bought the 47 inch wide material. I put Dynamat under mine.
If you really want simple and fairly cheap call you're local "industrial" gasket supply company (like me)....they should have rolls of 1/16, 1/8 maybe even 3/16 & 1/4 Black neoprene rubber sheet material in either 36" wide or 48" wide rolls. They should be able to roll you off a few lineal feet for not a whole lot of $$$. OR if they cut gaskets they may have a scrap piece lying around that cut something else from that might be enough material that you need they might sell really cheap or possibly free!!!
If you want something besides rubber go to Home Depot and look at their indoor/outdoor carpet. I used it several times for the trunk floors of my cars. It comes in several colors.