Yes. For your application, repainting the "W" on the speaker panels, I think frisket is just fine for that. The people saying to use different types of hardboard or other kinds of thick material might think you were doing cabinet art. Using the hardboard or other thick materials is good for that because you get that super slight overspray look that is a factory correct finish. But that only really applies to cabs that were painted that way from the factory(i.e. almost all EM's and most pins up into the start of the SS era). B/W started screen printing cabs before WW was made so that's why companies just make replacement decals for them these days.
If you've already redrawn the artwork and can print it out in black(for the high contrast), you can use that to cut out your frisket stencil, then apply it to the grill. If the grills are removable, I'd remove them to make it easier to work on and not have to worry about overspray.