Whoooo I wish I saw this older post from 2017 earlier. I yanked my rear headrests out last year when I bought my car, but never thought about covering up the holes.
What I learned from this thread: 5/8 is too small, so loose and could fly out. 11/16 on the big side and needs to be mooshed in. So i went right in the middle and bought 16mm plugs on Amazon. Just popped them right in. They seem very secure but I can still get them out with my thumb nail. Win!
Here is the link to what I bought:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
I had to buy a bag of 20 for $9. If anyone is in Bergen or Passaic county in NJ and needs some, I got plenty I can give you for free.