I used to use a common hole punch to make a little disc from black electrical or black gaffers tape and use that to cover the toner windows because I found that

I used to use a common hole punch to make a little disc from black electrical or black gaffers tape and use that to cover the toner windows because I found that Sharpie just wasn't opaque enough in all cases, especially with my Brother color laser. And then I discovered the paint pens my wife uses for rock painting. Now, those really work.