I must confess I am a lazy gardener. I don't like to start my tomatoes inside and then transplant them outside again. I often just plant the seeds right in the ground. I have used cut empty milk-jugs for mini-greenhouses before but I didn't have any this year so I re-used the jugs that I had collected sap in to make maple syrup. The milk jugs worked well in the past so I suspect these will work well as well. I should have maybe cut them to use one per plant rather than two, but, being the lazy gardener I am, I have faith that it will all work out and it usually does. :)
I have my complete garden in already this year which is a head start! It has been so warm this year already. This is why I really got to thinking about little greenhouses in case the temperature dips. I put the seeds into the ground, watered it and plopped the container on top. I will keep an eye on them once they pop up to make sure they don't over heat.
Here is an article about using jugs as a complete el cheapo greenhouse. http://www.greenphonebooth.com/2012/07/how-empty-milk-jugs-changed-my-garden.html There is no need to spend oodles of money on a greenhouse if you don't want to!
(Photo taken from another milk jugs as greenhouse site)