about to build the enclosures

Well it depends on what subs you are using on what a difference in size will do to the response. Typically sealed enclosures will not have too much of a change in response with a small difference in volume. If you are asking simply because 1 cubic foot is simpler to calculate internal volume for then I would say that the subwoofer displacement would make it closer to 0.88 cubic foot if you were to make the enclosure 1 cubic foot large. There are many places you can get the trunk liner at. Even stores like PepBoys have it that you can buy. Another place is parts express. You can find it on the link here. They also have enclosure carpets that are pretty nice. If you want a close match to OEM then I would go to a local upholstery shop and see what they have. The number of layers you should add on top of the fleece depends specifically on your install. Every shape/install will need a slightly different amount. So there is no typicaly amount that I go for. I just add more and more until I am satisfied that the enclosure will not flex and will be very strong. With a smaller enclosure you may not need to add many layers to get the strength required.