Don't use metal cans is one suggestion, I've seen one guy that does it kinda like u but he just takes the clone and puts it in a beer bottle with just the tip of the stem in water like that but here this is how I do it and I almost always get roots, my problem is keeping em alive after I put em in soil!!! I put my clones in a plastic zip lock bag to increase the humidity levels while it is tryin to grow roots. Oh and with the roots needing darkness I found a cheap easy cloning medium, 50/50 mixture or perlite and vermiculite works well for me.