If you search the forum people have given tons of links to good magnets. I have some that are bars and some that are discs. They each have good uses. I actually stack two up on each other, then wrap them in duct tape to make a little tab to pick them up with.
They’ll hold 1/8” stuff really strongly (will flatten warped plywood easily), 1/4” stuff much more weakly, but still enough to hold things down. The bars work well as “chucks” to seat firmly against straight edges of material if it’s already flat.
I use a steel bar attached to my workbench to keep them handy and prevent them from all coalescing into a giant megamagnetball.