Siva....I don't know anything about "simple plastic" designs but I do know of a couple of ways to make very simple planes.
First off would be the "RCH Trainer", "RCH Edge 540 Profile Plane", and the "RCH Biplane". You can find free downloadable plans in the Store tab of this site. I've built all 3 planes and they all are very easy builds and they fly very well. The RCH Trainer is a very stable high wing trainer plane and it's just awesome. The RCH Edge 540 is a plane that I had a hand in designing. It flies real well with a 2S battery and it's very exciting and fast with a 3S pack. The RCH Biplane was designed, originally, by me and Tony helped to refine the design and he made up the PDF plans. All three planes can be made from sheets of foamboard but I imagine that all 3 planes could easily be made from coroplast, as well.
Secondly, Check out "Experimentairlines" on YouTube. His has dozens of easy to build, and affordable, build ideas and easy to follow build techniques.
Best of luck!