I think I would take out the mattress and build an inside frame with a strong inside pad that is at 90 degrees to the slope of the panel, braced where and how you can.
Then I would match that angle with another frame, perhaps hinged off a house or garage. Put a piece on the frame so that you can apply pressure from the frame over your outside pad, maybe using a hydraulic jack or two. Build it so you can apply shims if the pressure isn't even.
It might work. The other option that I often see is putting in a whole new front piece. Likely that would work better.
