When I do these slots I lay some removable drywall tape in place and start wailing away with the hacksaw. Well. Maybe it’s one stroke at a time. So it’s not really wailing. To do the opposing side of the crown, I take the same piece of tape and invert it so that I’m using a clean edge for a line of sight. Saves on tape that way. Next step is to run my little Bahco 6″ Barrette files through the cuts and show them who’s boss. Cool shit, huh.