My family runs a bottled water company in Pennsylvania. I recently asked to bring home a dozen "leakers". They are 5 gallon water bottles that are no longer able to be sanitized and redistributed due to age and condition. Each of these bottles costs more than you would expect, and are reused countless times before they are ready to retire. Upon removal from the bottling line they are crushed, bundled and sent to a recycling company that regrinds them and then the plastic is sent out to become a new product.
I decided to take a chance with this unseasonable warm weather and plant out my Tomato and pepper plants. Each plant has it's support stake in the ground, and after cutting the bottom off each bottle, they fit nicely over the stake and down to the soil. I removed the cap from the top, to let hot air escape, and to water if needed. The height of the bottles in contrast to some other suggestions is greater, allowing the plants to remain under cover longer as they get taller. So far, we have had lows in freezing temps, and a daytime high of 85. The transplants are thriving!! This a full month before my frost free date!
I would be willing to bet that if you have a bottled water company in your area, you might be able to pay a nominal amount to get the "leakers" and make your own little green houses. I simply used a box cutter and carefully cut around the lowest rim. Another bonus is that the polycabonate bottles are very durable, the same material used to make certain greenhouse panels. They are easily stored stacked on a tall plant stake pushed into the ground.
So far, this experiment has been a huge success. I hope this post offers some useful information for fellow gardeners.
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I have used 2 litre Soda bottles to shield small plants that I have planted in the ground, fastened down with wire hooks made from electrical fencing wire. We have such good drinking water here in Iceland that these large water bottles are rare to find here. This is a good tip that works in colder zones. 
