I did it! I caulked the shower basin! Now I kind of wish I had taken out and redone all of it to match… but the most important bit is done. I also realized at a certain point that laying down painter’s tape would have been a good idea to help maintain the overflow (I tried cleaning it as best I could with tissue and sponges), and I should have covered the bottom with papers or plastic to catch the drips and strings and globs. Learning experience!