My wifes SGS2 has had the annoying echo only when a cover is on.
I tried the business card over the back of the speaker. Did not work....
Then I studied the picture in the post again - toxicdust outlines in red in one picture the area to put the cardboard - then in the picture with the cardboard in place the cardboard extends further - so it covers the gold spring/contact to the right of the speaker (the picture just under step 7)
So I made my cardboard piece a little longer - approx 16mm so it covered the speaker and the gold contact. Now the echo problem is gone completely.