I'm doing this exact conversion right now. It's been on my to do list for ages, but the rear panel always stumped me. It had been suggested that cutting the panel in three, on an angle just inboard of the tail lamps would work. It does, sort of. But then you have a seam right through the ribbed section, not easy to correct. I had a couple of spare panels, and I tried it out, just not satisfied with the result. Current attempt, and this one looks like it's working involves cutting the panel into 5 pieces.
