Different method of attaching ceiling

Current build for a customer required white fiberglass shower panel as a ceiling, in a light weight camper. So with trying to eliminate any extra wall thickness and weight, we decided on 1/2" ply walls which made routing my usual rabbit for the ceiling to nest in out of the question. So I laid the panel out on horses, attached the spars to the top side with adhesive, and laid the whole panel in as 1, attaching the spars to the side walls as we moved up. Worked great!
