I do not have a good pic of the styrofoam planters I have made but what I do is.....
-I use cheap outdoor caulking.
-Spread it on the container like icing a cake
-Spread out gravel on a piece of paper and roll the container on the Caulking.
-Turn and do the next side.
-Finish the top rim with some caulking and more gravel.
I tested this with some gravel from a old aquarium that I was not using anymore so the gravel was free.
