I did a forum search, but I did not find anything. I think I remember something like that as well, but maybe I read it in a thread that was not directly related to the topic? Maybe I just did not look hard enough. However, I figured having a forum thread that is hopefully easy to find would be good for me and others.
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A search on “ferrous” turns up several hits including one from @dan confirming that they went out of their way to use a ferrous tray (including the one @markwal just posted as I was typing this.
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As far as restrictions height is the only one I can think of. Keep it under a 1/2" from the tray.
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I’ve been frequently using rare earth magnets to hold down the lighter materials and to address slight warping in materials – they work really well.
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@karaelena made a hilarious video post that showed the head sweeping over the work and sucking the magnets off of the bed. “Now you see 'em , now you dont.” 
So strong magnets are good, stronger than the ones securing the head.
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Would something like this work pretty well?
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Just remember to wrap a tape strip on them with an inch or so overhang for a handle. Can also mark them on one side so know how they go back together, and avoid a finger nip when they get close together and squirm out of your grip.
Some of them are strong. Really strong.
If you have an old hard drive laying around to destroy, they also have a nice flat rare earth magnet inside.
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The strongest magnet I have came out of a terabyte drive.
Then there are those that Amputate…
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Did someone say “ferrous”? Now we will remember what to look for in searches.
I use some 3/4" neodidlium magnets put on a flat steel bar to make them a bit more versatile and hold even stronger. Haven’t had any issues using the magnets except when I’m off with positioning and there are some shiny sparks.
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I wish you had posted this idea before they bit me on Saturday. “Bad magnets, down boy, down!”
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Neodymium magnets will work, but they are overkill.
I bought these when they were on sale awhile back, and they work great!
Mmmm… I’d say it really depends on your need. I’ve absolutely used them on some thicker cardboard where I’m not sure it would have held if I wasn’t using neodymium. That air assist is surprisingly strong.
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Agree. Ceramics sometimes work. Neodymiums always work (if magnets are going to work at all). And Neodymium magnets are cleaner - ceramics can leave dark marks from the coatings used on them (or the material they’re made of). No value to a sometimes solution when an always solution is available.
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I usually do just the small-to-medium, but I’ve had some pieces of wood and cardboard that just wouldn’t flatten out until I broke out a bunch of 3/4" Nd magnets with steel washer backers.
Not sure most people would consider a ceramic magnet a “rare earth” magnet. Typically rare earth are Neodymium and they’re much stronger than ceramics (which are usually some powdered ferrite like strontium ferrite pressed into shape). They’re not made of any of the rare earth elements. The title of this item on Amazon is a bit deceptive.
It may be a technical point but you’ll get far more holding power per unit of size with rare earth magnets (neodymium or samarium) then you’ll get from ceramics. Rare earth ones will tend to demagnetize at lower temps though - not usually an issue in our uses but a downside (in addition to cost differences).
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We don’t recommend putting anything in your Glowforge unless it’s laser-compatible. We recommend using small scraps of Proofgrade™ material if you need to weight down the edges of your material.