When building, I usually try to use strip or sheet stock that closely matches the size of what I am attempting to recreate. Since I work small, I generally do not have thick stock on hand. Luckily the Evergreen sheet laminates well. Make sure the surfaces you are gluing are flat and free of debris. Sand or file the surface to get a smooth flat area to glue. What you don't want is areas de-laminating or showing cracks when they are cut to final shape. Using Testors solvent liberally coat both sutfaces and press tight together. The solvent dissolves the surface of the plastic and if you do this just right, the seams are near impossible to tell.
Here, a few parts blocked out in multiple sheets of suitably thick plastic. They will be an axle support, the main trail and cheek pieces for a large Parrot gun.