The old timers used organic moss in the box to trap gold and ancient miners used lambs fleece so anything that has spaces for gold to become trapped in will work, even carpet will trap gold.
Of course some furnace filters will work better than others. Definitely stay away from spun glass or foam or paper filters for obvious reasons. But other materials especially waterproof ones with a coarse enough weave should work fine.
I'm just not a fan of using anything that makes clean up more of a chore so I seldom use miners moss or carpet in any of my sluices.
Uni sluices such as Le trap, Angus Mackirk, etc., Drop riffles, vortex mat, magic mat, or fluid bed traps like the Bazooka Gold Trap are the way to go because you get all the gold without trapping a bunch of worthless sand and gravels.
GG~