Filmtools, Inc. in Burbank sells 16-oz Duvetyne (which they list as "AKA commando cloth') for $7.85/yd in 54" width, cut to length. You might want to call and ask them how opaque it is; I have some, and it is really black in reflection, but I can't vouch for the opacity.
You may also find that black poly backed up with Duvetyne is both light-and cost-effective; the poly is cheap but may develop pinholes, but the cloth should easily soak up whatever comes through a tiny hole if the layers are not in close contact.