pvc pipe would work just fine i think. i would be careful if you use corrogated black plastic drainage pipe like below. though it is probably your least expensive option and it would be strong enough if you got the rigid variety ,but it would most likely cook your plants roots if in full sun for for any length of time. a shaded area would work very well i think and have an interesting industrial look to it.

youre best option all around would be white pvc, as my father often used that in his greenhouse it was very similar to this and is often used in professional greenhouses and gardens. though i noticed after a few years time outside in full sun it would sometimes get a chalky coating to it as the outer layers weathered.