Is there a right/wrong way to position it?
There are a couple of thoughts here:
1.) Heat pipes behave differently depending on their orientation. Horizontal is a neutral position. Vertical where the re-flow of condensed water is aided by gravity can improve their efficiency. Of course with the NH-L12's 180 degree bend, once the fluid hits the turn then its acting against gravity. I'd say that in this case, horizontal will likely be better.
2.) Fin orientation with respect to airflow. If you have a top to bottom flow, having the fins oriented parallel to that will be most beneficial.
Given those two points, I'd probably recommend orientation 2.
To all that have built there case, how did everyone install a 120mm fan underneath the PSU? there aren't any screw holes
Double sided foam tape.