Carpet is very good and thanks for reminding us CKR. I always ask do you put the backing out or the face out? I find that shooting into the backing works better for me
I don't leave a space between layers but that's a good idea too. Get an old discontinued sample binder from the local carpet store and use the rings or the big double ferrule screws to bind them together. I also put some glue between layers. The helps keep it dry. Then a garbage bag over the whole thing. As long as the top of the bag is in good shape, it will keep it pretty dry.
If any of you get the sand tubes for your pickup in the winter, those bags make great target bags. After the winter, dump the sand in your garden (good for the garden btw) I've stuffed them with plastics, carpet and old nylon clothing, comforters and similar junk and have created a mini 3D course that sets out in my yard all year. I sew the bags closed, (even the U Stuff It velcro does not hold so I sew it -- nothing fancy -- 1 inch stiches with a big leather needle and some stuff like serving or leather repair thread) That allows me to create a folded over hole on each end, run a string thru it. I can hang these on a tree or some other likely object and shoot from one to the other. Some are flat, some uphill, some down, some longer some shorter some bigger or smaller. Spray paint a bullseye on the bag too. If you hang 5 targets and reverse the course, (shoot coming and going), that give you 10 different shots. Cheap targets too.
I guess I'm a little nutty about this. But it's fun.
sam