Hello Thomas
thank you for your report. I will investigate upon my arrival at my shop this evening. As you know, the Plastic Stern pieces might need to be shaped to "match" the same shape as the last stern frame pieces. Do you have your Plastic Stern pieces glued to the template base? If not, you should soak or run warm water which makes the Plastic Soft and manageable, and use your hands or fingers to shape the plastic pieces to match the shape of the wooden frame pieces at the stern. If the Plastic Stern pieces are already glued and installed, you might use a very warm hairdryer of sorts to help make the plastic soft.
However, I will at your request inspect the wooden part to see if there is a discrepancy.