The whole container. It had a snap-on lid which has died of old age.
It got brittle from years of weather exposure. Holes were drill in lid and bottom. But clear plastic tarp could be taped on, holes punched. Lids is removed after germination and temps are appropriate.
Most of my stuff is wintersown in milk jugs. But sometimes I sow very frost-tender stuff into cups, because most annuals will grow fast when weather warms and I don't want to have to pot up a bunch of seedlings while waiting for no danger of frost.
Karen