How does one evaluate the sharpness of a fisheye lens throughout its field of view?

While I have been analyzing sharpness and other optical properties of rectilinear lenses for a while, I have received a fisheye lens covering 180° and am having trouble producing an adequate test. Center sharpness is obviously easy to measure, which is most lenses is fairly good these days and so the main difference tends to be on how much softer are edges and corners and how much the improve while stopping down.

With the fisheye lens, it is difficult to get a target covering 180° field-of-view what is at edges is really distorted making it hard to make a comparison using what appears from outside the target. Bonus points if the method also applies to measuring vignetting. I know another major website that reviews and they show sharpness but not vignetting measurements for fisheye lenses.