You do supposedly need a high 'air to surface' ratio to help the Scoby 'breathe'. I don't use the suggested bowl, but a large sized anchor hawking 'cannister' type jar (mainly because the top rim is PERFECT for holding a rubber band to keep my 'tea towel' in place.
Most places I've read for what to cover the top with suggest one layer of a cotton cloth (kitchen tea towel or a birdseye flat fold diaper work great), so that it keeps out fruit flies and dust, but allows some air circulation, plus the towel hanging down over the container keeps it nice and dark and away from direct light. The rubber band keeps it in place and will make the top nice and 'taunt' so that the towel doesn't hang down in your tea...
Hope that helps some. I would not use a solid barrier, the tea may brew fine, but the air helps the newly forming SCOBY to stay healthy and aerobic, as you NEVER want anerobic stuff growing in your Kombucha brew... and it will tend towards the 'vinegary' side with less air circulation. (But that is a personal choice).