Depending on decorative container size and where the arrangement will be placed, you could try this: use a plastic design dish that will sit securely atop your container. Tape appropriate size brick (or part thereof) of foam in dish and design away. Be sure to hide your mechanics well. To transport: we flip an identical sized design dish upside down (gives a little space for the draping greenery), put the dish with arrangement on top, and secure with pegs in our beloved Seminole Delivery system. Fill decorative container with sand (or other material) for weight and stability. Put florist fix on lip of decorative container. At site, place design dish on decorative container. Don't use florist fix if container is a customer's heirloom or paint could be pulled off with anything tacky/sticky. good luck!