I'd mount it on the "front" side... you have some nice deep grooves for roots to grab on to. A hint when mounting... Mount the plant directly against the cork. Then, if more moisture is needed, you can put a small pad of sphagnum on top of the roots. But having nothing between the roots and the mount encourages them to attach to the the mount instead of the moss.
Just wanted to share how it is going. It has rooted in well and grown a lot since I mounted it. I'm still a bit worried about the stability of the cork in the pot but if I can get my hands on a pot clip I will probably stick that through the cork which will be perfect.