BTW, the fundamental construction between a gen 16610/16800 and 1680 are more or less the same; you have a crystal that seats to the rehaut lip (either with the entire crystal as would the case with an acrylic, or with the gasket and sapphire) and then you have a retention ring that seats over the crystal or the gasket. So, keep in mind that the new sapphire crystal should also have a small channel on the underside to seat with the gasket. However, with these factors in mind, you should have a decent, water resistant retrofit
May be worth checking with some of the crystal suppliers like GS, or watch supply houses to see if they have something in stock that might work...