On the first picture, the half on the right, on the side facing us, this is like a tongue and groove joint that goes on the inside of the other half when glued. The other half should look just the same and they will fit together like a puzzle after you cut them out. When you cut them, look for a VERY faint line and that is where you cut. You will roughen the plastic with sand paper in the lower area on the right part and the inside of the other half.
dan