Here are two ways to make hoop houses which are inexpensive and very
effective:

http://www.freeplants.com/how-to-build-a-hoop-house.htm

http://users.ntplx.net/~davesas/ghouse.htm

If you build them low-to-the-ground (3-4 ft high at midline), they retain
a lot of ground heat and do a nice job of keeping hardy plants growing.

I have versions of each type. The PVC pipe hoop house is a little more
work to erect to easier to manage in the long run since you can get at the
plants without having to right your way through the smallish opening of
the concrete reinforcing mesh.

I anchored the PVC pipe to the 2 x 4 frame using hardware that is intended
to securing piping in place--wish I knew the name of it.