Built sub box, what to cover with?

[89chevy]

bult a downfiring sub box for under the rear seat of my 2000 ecsb. Very happy with how it turned out but now the question is, where do i get the right color fabric to cover it with? My truck has that medium pewter color interior. Also, what is the best way to hold the subwoofer in without worrying about it vibrating loose and falling out being it is downfiring?
Thanks