As long as the exact shape isn't structurally critical, I'm okay with symmetry. I would use a piece of paper big enough to cover the entire opening, fold it in half, transfer the line I have onto the paper on one side, then transfer the line onto the opposite side of the paper, then draw a line to connect the ends of the lines that makes sense and cut it out etc... I'm collecting photo's off the net of examples so I should be able to get it almost perfect.

I just wanted to get some feedback about it in case I hadn't thought of something.

Thanks for responding.