I'm making my own as well. For heavy water I took clear hot glue stick and stretched it to get it skinny enough to fit in the tube, cut it to size, then painted it with a clear blue sharpie. For the meteorite I used sand mixed with table salt for color and sealed the bottom stand of the tube so it wouldn't spill out. For the radioactive element I took shattered safety glass found in the sand and combined it with a piece of clear plastic from a pc case LED light tube dremeled to crystalline shape, shattered safety glass or rock salt should work fine on its own though. I then colored those with a red sharpie. The water and crystal elements allow light through and give it a nice look.
