Hi
This ?? is right up my alley lol I've used the great stuff in about any situation you can name lol
"great stuff" comes in 3 degrees of expansion use the least expanding as it will have less voids .
It is very bouyant so either form it on a rock or glue it to a rock after forming much better than gluing to the glass as it can be moved
Tends to let go over time to very smooth surfaces such as glass anyway. carve after curing.
You can cover it with cement if you like or add cork, rock or what have you but not necessary .
I use acrylic latex found it holds up just as well as two part epoxy at about a third the cost Have found a type called "patio paint"
available in small bottles at craft stores . Comes in 80 colors so you could put the "mona Lisa "on the side if you like lol
No the foam does NOT disintegrate over time and both the foam and the paint are non toxic. It will degrade in natural sunlight but easily prevented with the paint.
Will warn you that certain fish such as CAE find it delicious,will spend hours scraping it off . You can attach non strata plants such as Java moss fern to it but roots will grow directly into it causing a bit of paint chipping
Be sure to wear disposable gloves !! It is incredibly sticky until cured and you'll have to wear it off as nothing removes it gary