I think it would be nice if you made like an HSF (without fan)on the piece of copper, outside of the w/c circuit
some pics are coming
[edit]here is the pic
i'm very bad at designing

what i'm trying to explain is that the graphics card would look funnie, smarter, etc, if you make some heatsink maze, and, by the way, that would increase the surface of the cooler... with the low airflow in your case, that may help cooling you gpu
[edit2]second pic :
a more stressing maze, but not too hard to mill (i think)
here is what i think, but do as you want, that's the way for w/b _you_ like