I gave up on demagnetizing, and just keep lumps of White Duck Poster Putty stuck to the column of the drill press stand. It removes anything from the graver including the irritating small dust stuff after sharpening.
If you have an electric soldering gun, you can pass the graver through the loop!
Over 40+ years I have had to demagnetize vise jaws, engraving block jaws, handpieces, large fixed blade knives, steel belt buckles, odd shaped pieces and parts of various machines - you name it.
I seriously doubt you could demagnetize a standard 2" x 2" practice plate with a demag lap, but maybe?
I don't want to take the time to play with one to find out.
I have the same one you have Sam. Used to use it many years back. (need to put it up for sale, just collecting dust in my shop)
It WILL demagnetize anything you can get through the hole. Therein lies my probem with it... that hole just ain't big enough.
And you need to fix/attach it to the bench, or have 3 hands to use it for anything longer than a coupla inches.
Pressing the button while supporting both ends of whatever long thing you are trying to put through the hole is more'n a bit awkward - if you don't mount it to something.
I would like to see this work. I've tried it and it did not work, despite having read for years that it does. The only way I could get it to work was to pass the graver lengthwise over top of the body of the gun while I pulled the trigger. You can feel a slight magnetic pull and the graver demagnetized perfectly. But passing it through the loop never worked for me, and I wonder if it really does or is it an old wive's tail? You'd think Youtube would have something on this and I couldn't find a video showing it. Makes me wonder.