Dig out your old structural design textbooks , and see if you can get a rigid stucture that supports your GEM, counterweights (cantevered) and your OTA with incorporating the uPVC tube as a pier.
The mechanical properties are here :
http://www.boedeker.com/pvc_p.htm
You'll be looking for near zero flexure under load , and near zero bending too , if you hope to achieve good tracking performance from your GEM.
This is the reason why most go for solid concrete , steel reinforced concrete , or thick walled steel tubes as the basis for their piers. Steel being the best common metal.