Sorry to be slow to respond. I totally know about batting, but I decided not to use it. It does nothing for comfort and is designed to make cushions look plump and filled out. I decided to skip it, and go with a more squared off look. Plus, I am gonna put a fitted sheet OVER the denim material to keep it clean and to make it softer.
Anyway, I finally finished covering the mattress. The pictures are of the mattress leaning against my wall, so it looks a little more "bunchy" than it is. It came out really well I think.
I used a 100 inch zipper (sleeping bag zipper) so I could get the foam in and out. It was tricky to make the zipper make the corners, but it worked!
Fun fact... I used an entires 150 yard upholstery thread spool on the two cushions. And went through a whole pack of number 100/16 denim sewing machine needles. All on a 1967 Pfaff 79 I got on eBay.