Mustang

Isaac, that's exactly the same method I use for my seedlings. An inverted plastic cup over another.

I use pop bottles as well, especially for deep rooted seedlings such as palms. And they also make great packaging containers for posting plants. I drop the plant complete within it's plastic cup, into a pop bottle that I've cut open part way. Seal the pop bottle back up with sticky tape, pop polystrene balls or packing beads in through the top, then screw the top back on. It keeps the plant humid, protected, and easy to post.