FRP panels to replace the plywood
Smitty, Home Depot and Lowe's are both selling FRP (Fiberglass-reinforced plastic) panels that are perfect for this application. They come in 4x8 sheets (HD will cut in half if you need smaller), have a shiny side with an imitation leather-like pattern and a flat back, can be cut with sturdy scissors, are rigid enough to hold their shape on the ceiling but lighter than plywood, won't absorb water, etc. I cut out new ceiling panels from this stuff using my old plywood pieces as a template and decided the pattern was good enough that I would forego the vinyl covering (but you can still glue the edges of the vinyl to the backside of these if you want to). The panels come in white or almond, and if you want to see what they look like I think the Home depot stores even use them as paneling in their own store rest rooms. About $27 for a 4x8 panel...
