Well, here it is finished, but not hung yet...
Made the frame out of pine, which also has a piece of drywall mounted to the back side. I painted the frame to match the panels, and the drywall to match my wall color. Then I mounted the panels from the back side. This should allow me to hang the whole thing on the sloping wall and the eye should be tricked into thinking the panels and frame are mounted directly to the wall. That's the plan, at least.