I carry Rubbermaid tubs with me to fill with water on site. Don't forget to take your florist food with you to mix in the water. Allow the foam to "float soak" - meaning you let it sink to the bottom naturally, not forcing it under the surface.
That can cause dry pockets that can mean death to a flower stem.
I press my foam lightly on top of the container, imprinting the sides onto the foam brick.
Foam is easy to trim to shap with a sharp, flat bladed knife.
A few quick cuts and the foam is ready to fit into the container.
Fill your container completely full. Large arrangements like this need a lot of support, both in water and foam. Too little foam and it can begin to degrade and crumble as more and more flower stems are pushed into the foam.
Cut pieces can be fitted around the block, filling in the container completely on the bottom.
I'm going to place another brick on top, so that the edge of the foam rises above the edge of the container and urn.