Home Depot & Lowes usually sell outdoor carpet in 6' widths. They'll cut it to length for you, so you'll get 6 x whatever, then use a razor knife to trim to fit. It can be very inexpensive and they should have a few choices of styles and colors.
Also you could try a stereo/ car radio parts outlet as they should sell the heavy felt-like product that speaker box makers use. That's a good choice, too.