Just discovered this -- it contains the optimal constructions for up to N=20. They don't prove optimality but they do claim it. https://www.maa.org/sites/defa

Just discovered this -- it contains the optimal constructions for up to N=20. They don't prove optimality but they do claim it. https://www.maa.org/sites/default/files/pdf/upload_library/22/Robbins/Patterson1.pdf https://www.maa.org/sites/default/files/pdf/upload_library/2...