I'm going to actually fit flower foam down into the glass liner. This assures the flowers don't move and accidentally slosh water in between the glass and the cardboard box.
I press the glass on top of the soaked foam to have a guide to cutting my saturated foam to size.
Press hard and your vase will leave an deep imprint as a guide.
I cut a round cylinder shape and slipped it inside the vase. It was a pretty tight fit and you can see I had to press down hard to fit the foam snugly into the glass.
I left the foam just short of touching the bottom - I'll explain why in a minute. Notice that although I usually leave my foam above the vase or container lip, this time I definitely left it well below the top edge of the glass to be sure all the water stays INSIDE the glass.
The centerpiece will have to be rewatered each day. Had I put the foam completely to the bottom, it would have been easy to overfill and spill over the foam, overflowing the glass.
Instead I leave a short, open place at the bottom of the glass that isn't filled completely with foam. I take a long stem or a Cowee pick and push it down deeply into the foam, piercing it completely through to the bottom.
I create a second hole the same way. Now when the centerpiece is watered the water will funnel quickly to the bottom of the glass and the foam can soak it up from there.
If you take care when re-watering the centerpiece, the excess water should always flow to the bottom of the glass rather than overfill and spill over the top edge of the glass.