you could use a plastic laminate! a guy i worked with did a box and covered it with some black lam. that looked just like leather. was way cool looking! but i say just color match the carpet to the carpet....i bought some carpet off a site online that is the exact color match and wasnt a bad price!

 

I just sealed mine with silicone from the inside, used a router to round all the edges, used body filler to smooth the outside, primed it, rolled it with silver rustoleum hammered for texture, then topcoated with a paint to match the interior. Looks pretty good IMO. I also used foam weather stripping to seal the woofer.