This looks like something that a do-it-yourselfer created and not a HVAC/R professional. Bottom line here stop being cheap and have it done correctly the first time. Acceptable materials for plenums are sheet metal and ductboard. Dont take my word for it lets look at the International Mechanical Building Code:
602.1 A plenum is an enclosed portion of the building structure that is designed to allow air movement, and thereby serve as part of an air distribution system. Supply, return, exhaust, relief and ventilation air plenums shall be limited to uninhabited crawl space, areas above a ceiling or below the floor, attic space and mechanical equipment rooms.
**602.2 Construction. **Plenum enclosures shall be constructed of materials permitted for the type of construction classification of the building.