I would agree about not using the copper. The corroding can be pretty, but the problem you will face is where it touches other metals (the rack, the screws you use, etc). The interface between the two metals will result in a galvanic reaction that will cause "corrosion" at a much higher rate. Thus you need copper or materails that have a sacrificial zinc galvanization to protect them from the reaction.

PVC or that old stand by wood maybe more cost effective and easier. The loads you apply to the pvc are trivial so it may not matter if it gets brittle after a year or so. The cost of replacement is so low even if it does happen to break, replacement is not an issue. But, pvc will break easier on a cold day with an impact and most scheduled pvc are designed to be covered with dirt or are interior to protect them from UV. Maybe the grey conduit (unsure if it is actually pvc) would be a better choice if it is stabilized for UV.

Eric