I think this has just opened up a floodgate of ideas for me beyond just bottles. The first thing that popped into my head, that would make sense as an addition, one of the old glass buoy. The end of a sewer drain might be of interest to some as well... If my tank were larger, maybe an old port window or the old school deep sea diver's masks. An old cannon ball. a sextant maybe? Though on some of these, the metal is probably a bad idea. But perhaps with some skills and funds, some of the metal parts could be redesigned with a acrylic or something.