there is some stuff woodworkers use around here that dries Crystal clear and is as hard as a rock,I will have to find out its name first. One fellow did a rock pool scene in the top of a table ,.put cray fish Shel's rocks etc and poured this stuff in and hey presto a rock pool in the middle of your dining table ,it dries as clear as glass,I will stop in and ask when I`m out that way
Kind regards Peter