The trick was to "Block Inheritance" on the immediate parent OU for the child OU in question.
- Right-click the parent ou of the ou you want to apply your custom GPO to, then click "block inheritance"
- Apply your custom GPO to the child OU beneath the parent OU you just blocked inheritance on
- From (domain controller) CMD prompt, type: gpupdate/force
- Run GPO Modeling to confirm custom GPO settings are being applied
This worked for us. The only caveat is to remember that when you block inheritance on an OU, you prevent all GPO's above that OU from propogating their settings via inheritance, which means if you are relying on a GPO higher up in the schema for settings, you need to confirm they are still being applied to child OU's beneath the OU you've blocked inheritance on, as you may need to replicate these settings on the custom GPO you applied to the child OU.