Instead of making all those grille pieces, why not just make continuous length pieces that are the width and contour of the grille, then mark the location of the vertical pieces on the backside of these. Mark the location of the horizontal elements on the vertical. Take a hobby saw that cuts a kerf the sane thickness as your plastic, and saw half way into the pieces; on the front of the vertical, and the back of the horizontal. Essentially, making half-lap joints that lock everything together. Just a thought...