I use Oasis florist foam. The new Max-Life is better than ever and can hold a lot of water. Remember the point to using florist foam is to give the flowers a constant water source.
I do tip out the water before delivery, to ensure it doesn't slosh around in my delivery vehicle. Just make sure to replenish the water when you reach the church. Carry a long necked watering can with you for this purpose.
Don't skip the basics like flower food, a flower re-hydrator and a flower sealant after the arrangement is done. These products don't cost that much and will extend the life of your DIY flowers (especially if you don't have the benefit of storing the finished designs in a professional florist cooler).
Always read the label and use the products correctly. Too much flower food won't hurt your flowers, but it would be an unnecessary waste of product.
Always let your foam "float soak". Don't force the brick under the water surface. This could cause dry pockets in the foam which could mean death to some of your flowers.
The Oasis becomes saturated very quickly and will feel heavy when you lift it out of the water.
I cut one end into smaller slabs and put the in the bottom of the container.
The larger brick I fit into the top of the container, trimming it with a knife to fit.
Push it down firmly until the brick rests on the foam in the bottom of the container.
You will want a good inch or more of foam to rise above the lip of the container. This allows you to insert greenery and flowers from the sides and give your design a more natural appearance.
I usually secure my foam into the container with greening waterproof tape. Cross the tape in an "X" pattern across the top of the foam and tape securely to the sides of the container.