To answer your OP:- Although I've not done it on my own boat I've seen 'gravel' used but be specific in sizing and you need smooth stones. You can get gravel graded to specific dimensions. Sized from 'lemons' to 'oranges' and washed and dried will fit snugly in just about any void and can be removed and refitted easily. It is also possible to get such gravels in specific materials and the very best is basalt but it costs Ebony Black Pebbles from the stone warehouse 30mm up should be a start for you