I like the idea, but why use roofing material which will hang around forever. Try two or three layers of newspaper to make your "collars" or even a loose cone to put the plants and some soil in. It will degrade into the soil and feed the plant after several weeks.
I love the plant collar idea but it occurs to me that It might not be a good idea to use newspaper or other biodegradable materials because of moisture wicking. Moisture wicking can also be a problem when seedlings are started in peat pots, you have to make sure you bury all of the pot or tear off the top before you plant.