When it's fermenting this vigorously, you can take the lid off entirely. Don't worry about infection at this stage, because there's so much CO2 blasting-off that no nasties can blow in. This is the historically accurate method of getting those elusive esters and phenols which are true-to-style for this type of beer anyway.
However, beware if you have cats. I do, and they cannot resist frest yeast, so I haven't yet figured out how to create a protected open-fermentation area.
