Loose grit on a cookie sheet will get flat in a few minutes with loose 60 grit Silicon Carbide and an assortment up to 500 grit, for $15.
For large stones, you can get large commercial cookie sheets or a large marble tile or slab from Habitat for Humanity and make a dam with folded Duct Tape. It will contain the slurry.
I have a 20X18 in marble slab, I paid $2 for.
No need to trash a diamond plate, Arks will pull out diamonds.