They stand up when the egg actually emerges? There's no way my girls are standing under the coop where there's only 10-12 inches of height. I think it's more of a squatting position. I wish they were limited to a space where they needed to actually stand up, because the eggs wouldn't be in the hardest to reach place possible. Darn chickens!Tubs can be unstable, like when they step on the edge and the tub moves.
Bird needs to feel 'safe', that's why nests with sides and tops might be better.
Once they start laying somewhere else it can be hard to break the habit without confining them to the area you want them to lay and keeping them away from the area that they want to lay.
12 x 12 x 12" minimum....I like 14 x 14 x 16" .
Did you know that they stand up when the egg actually emerges?