"cup your ear (a la Robin Gibb)."
Robin Gibb ???
This is one of the oldest and most universal methods ever devised for staying in tune, - used by muezzins calling the faithful to worship, broadside sellers peddling their wares on noisy streets... you name the time and place, it's been around. It was still being used by singers of epics in The Balkans into the middle of the 20th century.
MacColl and Lloyd introduced it into the revival in Britain in the 1950s; it became their trade-mark, and the missile hurled by morons who felt the need for slogans rather than argument - "finger-in-ear" still infests any discussion on taking folk song seriously.
As long as you have learned and absorbed the tune it works like a charm for unaccompanied singing or performing in noisy conditions.
Jim Carroll