Just replaced a piece of the sill on my house; rain had leaked in behind the brick-built entrance steps. However, I didn't use any plywood or similar but cut the old sill at an angle and sawed the same angle on the new piece that I put in.
Made an outstandingly nice picture in the extremely complicated drawing program Paint, but you might understand what I mean anyway. The picture is seen from above.
Drove a couple of screws through the angle, seems to hold perfectly.