When creating tall flower centerpieces, you always need to think ahead to the delivery plan. Tower vases are too tall for most vehicles. It's better to construct the flower portion in a low container and then assemble the complete centerpiece when at the reception hall.
You'll need floral clay. It comes in both green and white . . . but I usually stick to white. If you are using a green container, then order the green clay. This versatile product comes in handy in so many different ways. It can fasten glassware to another surface much better than messy hot glue. It can be pealed off later to reuse the glassware later or return to the rental agency.
These are the containers I use. They are cheap and can be used over again. Made of sturdy high-impact plastic, the underside looks fine to any guest who happens to look up into the bottom of the centerpiece and is much less intrusive in the vase than the white handled Belle Bouquet Holders (even the clear ones).
Keep in mind you will do this AFTER the flowers are designed and you are at the reception hall. This is important because the tall flowers are much easier to transport than when they've got an extension rod sticking on the bottom of them.
Lightly pat the clay down so it covers the entire flat top of the extension rod.
Keep in mind that this container is going to be filled with flowers, so you obviously can't turn it over like this. It's best if two people do this step, with one holding the flower arrangement and the other pushing the extension rod up onto the bottom of the silver bowl.
If there's only one of you, place the Tower vase on the floor, drop the extension rod that has the clay attached, lift the flower arrangement carefully and center the bottom of the container over the tower vase. Set the arrangement down on top of the clay. Grip the edges of the bowl and push down firmly to seal the clay against the bottom of the container.
Have someone hold the vase for you while you twist the centerpiece just slightly. This locks the clay in place.
Once finished, your extension rod will now come down and the container (filled with flowers) will be firmly attached.
Before setting the container on top of your tall flower centerpieces, place four slight flattened balls of clay to the top rim of your tower vase. This will help seal the vase to the bottom of the container.