NM clamp for cable fished into box
After looking at the comments, the real problem isn't the knockout, it's a clamp that will work with access only to the inside of the box.  

Luckily, this is a solved problem.  There are plastic push-in NM connectors made to install inside the box, such as the Raco Insider:  



This will install in a standard 1/2" KO.  

This is a very common problem, so connectors that will work with cables fished into boxes are available almost everywhere electrical supplies are sold.  

Big box stores are basically self service, I am sure there are exceptions, but I wouldn't expect ANY technical expertise from the staff.  I've even seen egregious code violations in DIY videos on their official Youtube channels.