I got standard 20cm extenders from the Internet.
Cost some €18,-- a pair.
Did not even know there are different types available and they work perfectly

Something like that: http://i00.i.aliimg.com/wsphoto/v0/12927...nt-b-belt-b.jpg

Hannes
once: Green M3W; 2013
now: Red 4/4 Sport; 2011
and some practical cars for use in real life