If you are going to fly a Guillows model with rubber power there are many solutions to the cowl problem that give strength and adjustability without using plastic. These range from simple torque boxes and torque tubes that take the load completely off the cowl to reinforcing the plastic cowl with wooden parts. A solution using PVC pipe seems a bit extreme.
Here are some examples from past year's "G" Challenge entries. All of these models flew sucessfully:
Simple torque box on a 500 series Stuka. (H.Littman)
Torque box on a 300 Series Aeronca (H.Littman)
Torque tube on a 300 Series Cherokee (G. White)
Reinforced cowl on 300 Series Aeronca (O. Olm)
The variations are practically endless.
Howard