I took a U-bolt and notched it in 2 places, then I tie wrapped it to a compass. The 2 legs of the u-bolt followed the arc while the pencil marked the amount of sidewall I needed to remove. This was all done using the hatch as the reference.
Click thumbnails for larger pic.
Modding the compass:
![Image](http://www.teardrops.us/userfiles/40/Picture001small.jpg)
![Image](http://www.teardrops.us/userfiles/40/Picture002small.jpg)
The mark:
![Image](http://www.teardrops.us/userfiles/40/Picture006small.jpg)