If I bring down my aperture this way I risk a flat chin, because I need my chin muscles to pull my lips down. I don't think this is the right thing to do, is it?

With some practice (watch the process in a mirror) it is possible to keep your bunched chin.. If this does not work use two steps: move your whole embouchure down. If this causes a flat chin, don't bother, Step two is to bunch your chin without bringing the whole embouchure back up again. Don't forget to look in a mirror! You can also check with your tongue. If you can still feel your top lip after these two steps you are on the right track.