Shake It Pouches by Impression Obsession. And if you have the square set from Spellbinders, one of them fits it almost exactly (varies with the paper/cardstock. This is thicker cardsotck, so I opened it a smidge with the edge of a scissor. The magnet will stick to even a tiny group of iron filings. A really strong magnet is worth the extra cost, the average dollar stores set will not be very much fun at all.
The Shake It Pouches come with adhesive, but Iron Fillings are heavy and tenacious so I use Glossy Accents as a seal instead. Patience is required. Put the pouch upside down, pour in some filings, apply Glossy Accents, apply background cardstock, press the edges together (without picking it up). Set it aside to dry. Really dry lol. Make a cover mat, or cut a window in the card front. The pouches make a much lighter card than making a well out of cardstock.
UPDATED to add: I had no luck just "substituting" iron filings for shaker card ingredients because they are really heavy in comparison and very very small. So the adhesive sheets just didn't work for me. They need to be sealed in with something like Glossy Accents and then make sure there are no gaps : ).
And if you spill any iron filings, wrap the magnet in a coffee filter before you use it to pick them up. then hold it over the jar and remove the magnet. The iron filings fall neatly into the jar without having to be wrestled off the magnet.