Low cost greenhouse plans ($50)

[TWP]

This is fairly simple to build, assuming that you have the space.

http://doorgarden.com/2008/10/27/50-dollar-hoop-house-green-house/

Using plastic pipe is lighter, but you can also use aluminum tubing with the help of a tube bending tool

The lifetime of the plastic covering is limited and thicker plastic with "UV" resistant properties is recommended.

This is NOT something I would recommend for a post SHTF situation, since the plastic sheeting will likely not be available...