Every library has a shelf of books to be sold off for a cuppla bucks. Making up a collage is fun and the results quite pleasant. I find Aquadhere applied neat on the back. Sand the spikes of wood off the ply wall then dampen with a sponge. Stick the cutout in place then smear a coat of Aquadhere over the outside surface and squeegee with a wet sponge to change out blobs and bubbles. Less glue will leave a friable paper that peels away from the wall.