I have a 2006 SL350 and this year went on a six week European tour so needed to cram as much in as possible. The space behind the seats seemed too good to waste so I bought a cricket duffle bag for about £30. Worked a treat.
I've used AUTOTECHNIC in the past and they are fine but no cheaper than Europa who I now use. They are an official MB workshop so you get a MB stamp in your service book.