Originally I used a pedal extender from High Sierra Cycling on the right side. It was a 20mm one and worked well with the cheapy SPD clone pedals that I had. Later on when I got a set of Crank Bros Quattro's, I felt that I needed extenders for both sides. I bough a pair of 25mm extenders from the KneeSaver folks.

However, there is just one small problem. The Quattro's have no wrench flats and mount and dismount by using an hex wrench on the inboard end of the pedal. The extenders by both High Sierra and KneeSaver are not hollow. You cannot insert a hex wrench into the pedal. The KneeSaver people were not responsive to the idea of selling extenders with a hole as they felt that it weakened the part. So, after buying the standard extender, a friend who owns a machine shop drilled them out for me. So far, no problems...