Haven't seen anybody mention the Undercover UltraFlex. Obviously its the twin of the MX4 but rails are a slightly different design (brackets too), tonneau is attached to the rails from underneath the cover (I love this over the elevator bolts of the MX4), and its covered in carpet underneath the tonneau.
Oh yeah its got a 5 year warranty over the 3 yr warranty of the MX4
http://www.undercoverinfo.com/products/ultra_flex