Designing flowers for a wedding uses basic tools. One important florist tool is waterproof tape. I use this tape as the last step in making sure my Oasis foam is secure and doesn't slide around.
Your edges of your containers must be dry before attaching the tape.
The the tape taut and tightly over the foam bricks.
Criss Cross the foam, securing it from several sides.
The waterproof tape will be concealed later by the greenery and flowers. Concealing your mechanics is always the mark of a good designer.
Here's the top of the foam after being totally secured into place with Cowee sticks, packing and waterproof tape.
As you can see from a side view, there is plenty of foam above the container lip to insert flowers and greenery from the sides.