Milk jugs do keep the "environment" a bit humid and moist - good for germinating seeds. Closed, they also keep critters and birds from feasting on your seeds. I think your idea of keeping the containers in a shady location (not full FULL shade though - - -you want some dabbled sunlight)is the way to go. I sow in containers like this till fall.

Yes, dappled sun or early morning sun only with bright shade after that, and extra vents in the top of the jug.

Karen