I was at a retreat in Oct. and we had a speaker about Barn Quilts. They used OMD boards, used in making billboards. Painted with Marine (for boats) and Marine varnish to seal them. I would think treated outdoor plywood would work. I have a friend that has one and it is mounted on 2 X 4's then hung on the barn with brackets. I want to make one too. will have to wait for spring. too cold to paint outside right now.
Merry Christmas to all