Peggy, I'm wondering what this is like for machine quilting. I found some on Amazon after reading your post, and it's 3/16th of an inch thick. That seems as though it might be pushing the limits of what my DSM could handle by the time you add the layer of pieced fabric, and yet I know the nylon fabric on the headliner would glide nicely on the machine bed. It's an idea I'd like to try, but it looks as if you have to get quite a lot of it (on Amazon). Maybe Jo-ann's sells it off a roll in a more modest quantity.
Also, has anyone attempted to use it on an embroidery machine?
Your suggestion also made me think of Neoprene, which is not as thick, but is also sew-able and stiff. It's used for scuba outfits, so would definitely be washable. It comes in more colors than headliner.
To get back to the original question, I have made quilted sewing machine covers using my usual Warm and Natural in the quilt sandwich and a medium level of quilting. They stand up just fine. Here's one that I've probably posted on this board at one time or another. I start with a flat little quilt to make the quilting simple. The length is the distance from the front of the machine bed over the top and down to the back (+ an inch or two to make up for shrinkage and loss due to quilting) and the width is the side over the top at the highest point and down to the other side in the same way. After it's quilted, I fold in half, sew up the sides and make the box shape in the same manner as a simple bag, by sewing off a triangle at each side and trimming and overcasting the seam.
I should mention that the block I used was from a design offered by RaNae Merrill at our quilt guild meeting one time.