There are plastic NM clamps that get pushed in from the inside of the box

you can use the clamp on metal ones. simply attach to the wire a foot or two back. remove the nut, fish teh wire down, and then once the threads are thru reinstall teh nut. the screws for the clamp end up in teh wall, but no big deal as long as you already tightened them down and will not need to run another cable thru that hole!

"There are plastic NM clamps that get pushed in from the inside of the box"

Ron, do you have a link to those? All I've ever seen are the connectors that have to be snapped in from the outside, otherwise they'd be backwards and won't retain the cable from pulling out.

The part I'm familiar with is made by Sigma Electric Mfg. Co.

It is good for 1 or 2 #14, #12, OR #10 cables

Part number is 44740

They have an unbelieveably bad web site and you can't get to specific parts. The part is different from the normal snap-in plastic bushing and in fact has "this side inside box" stamped into the flange that goes against the box to prevent it from pushing through.

I haven't found them at a local distributer yet but get mine at a big box - I usually clean out their stock of 10 or so bags when I buy them. I use them a lot in old work ceiling fan boxes.

I think the ones I got were from lowes or HD. I even screwed up and put the first one on the outside of the box (I was installing recessed fixtures) before I realized that the clamp goes the other way.

The RACO "Insider" is the type I found at HD (or maybe it was Lowes). The little bags they come in are easy to miss on the shelf, so you might have to hunt for them. They're great for securing a cable you've fished into an old metal box.