For those not familiar with these systems, look at these videos:
I used a adjustable nordic style ski strap (not the usual Voile rubber strap), duct tape to reinforce the ski strap and make it stiffer and a carabiner and a old prussic sling. The advantage with using a adjustable ski strap is that it can be adjusted to different ski widths, and thereby lowers the risk for the ski (and binding/heel riser) to pass through the loop. I am also not using a ski hook as in the videos, instead I use the prussic sling that I just wrap around the skis and clic into the carabiner to lock the upper part of the diagonal (this is an older skimo race technique, some seconds slower, but it is a safer mount than the ski hook). The pictures, I hope, is pretty self-explaining of the set-up.