I like the dremel with cut off wheels, it costs about $10 in cut off wheels but you get the opening real wide, the dremel fits nice and close to the edge so no need to have a any guide. I found it also takes a lot l fewer wheels of you cut from the top, ie scribe and then work your way down. My first inclination was to cut through and get it from the edge but you end up cuting more steel than you need to if that makes sense.