I installed the Polks in the rear today. Fit pretty well. I just had to remove the spring clips from the edges of the hole (in the Elise), and expand the holes under the clips outward just a bit. Then, I put the spring clips back on, and the speaker fit.
Must say that pulling out the passenger seat was the biggest hurdle for me. I needed to figure out what tools would reach, buy them at Home Depot, and overcome the fear of cross-threading (which at least 2 people on this board say they've done). Worked out fine. Actually, the hardest bolt to remove was the front right, cause there's so little room to get an allen wrench under the seat there.
But the rear is done!
Definitely sounds a lot better. Can't wait for my MB Quarts to arrive for the front!