Thi is wonderful work - I'm currently playing with this technique myself. I've couched yarn before on landscape quilts and in that case, I didn't glue, just held it in place as I zig zagged along. I had a go with my cording foot but preferred my open toe applique foot (on my machine I have a walking version - Janome 6600 Acufeed). Just a thought if the cording foot doesn't work for you. Great job!