2 things. First, In addition to the missing cover I was missing the 3 clips underneath that cover that hold the center of the belly pan to the car. I replaced those and now the rattle is gone. Rusty's and his guide above are a correct fix.
Second, I was contemplating which way to go, replace the stock panel which I was told is $200, or go with the Z Speed diffuser/pan at $399. I'd have a hard time paying $200 for a panel I'll likely loose again so just to confirm I asked the parts guy to confirm the price while I was getting the clips. He said it's $167 (could be the difference in dealer parts dept pricing) and he said oh, it's been superseded to a new number...
"I'm say oh ya, do you know what's different"? He said "it looks like it now includes 5 nuts and bolts".
When I got under my car to pop in the 3 clips, I noticed there are 5 holes in the pan right in front of the holes for the clips that hold the missing cover to the pan. I bet Nissan is aware of this issue and made the cover longer to also bolt on and prevent it from flying off. If I decide to go this way I'll post the result.
Also this is just my opinion but if the stock belly pan has a cover for better air flow and to prevent vibration from pulling the clips out, I think the ZS diffuser should have a cover there also since they are claiming to have better airflow. It would have been easy too as there looks to be rivets there. They could have used bolts and had a simple flat piece cover the hole like factory
