Making the template for the adapter was pretty easy: a piece of cardboard (top) was taped across the bell housing, the outline of which was then traced on it. The cardboard cutout then served as a template for the aluminum plate, cut on a bandsaw. (A jigsaw or even a hacksaw will work, since aluminum is a fairly soft metal.)

It's not even strictly necessary to perfectly follow the bellhousing outline when making the plate - some people use a simple rectangle. That will work (with bolt holes in the right spots, obviously) provided the edges/corners of the plate don't interfere with other components in the motor bay.

The "traced" shape of the adapter looks nicer though, and ensures there won't be any clearance issues.