Morning Gareth.
Indoor or outdoor use?
I have a CLK63 and a CLK55 (W208) both are covered with Halfords large outdoor covers. The version on the CLK55 (kept outdoors) has lasted well but after 7 years plus it has worn in places and started to go green around the lower sections. I binned it last week and will be replacing it this week with another. The cover has straps that pass under the car front and back, on the rear strap I had to add it on the CLK55 as it was too short, not sure about the W209, it sits in a garage when not in use and the cover just secures well enough with the elasticated sections to keep it in place. If anything the cover is slightly too long and is close to touching the ground, hence the green mould like substance developing on the 55.
These covers keep the leaves from collecting in the wiper wells and rear boot/window sections, they guard against all bird dropping and they can be washed gently and aired if they get too 'crusty'.
Hope that helps.
Greg