When I want a precise(ish) mask I put a box or boxes to make about 6"/150mm high on the baseboard and draw the outline on the back of a discarded test sheet resting on the box without changing the focus or head height. Then to use the mask, I can hold it at (roughly) the same height during exposure.
Not as precise as John's method for very fine detail (which I must try) but generally good enough to make a decent mask for moderately detailed burning/dodging work.
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