That seems like a good way to protect the rhizomes until they get acclimated. I have learned the hard way to just pot them up using potting soil with no fertilizer in it, and I keep them under an umbrella for a few weeks, watering them sparingly every 3 days at first and then stretching it out to once a week or so by the end of August; and I also remove the umbrella after 3-4 weeks. That way they're already rooted when I plant them in the fall, and because of our strong summer sun and heat that's what works best for me.
But I think your milk carton idea is a good one!
