That wasn’t me but I’ve had enough florists leave an arrangement for me…. most of them use a clear plastic saucer (it’s not deep or tall on the sides) with the oasis, and there’s florist tape in an X across it to hold it down. If I am smart enough to bring a little parchment or acetate with me, I can cut some to fit before I put it on the cake. Usually I like to suggest that the florist just leave me a bucket of flowers to arrange when I get there. I use a ball of fondant scraps and stick the stems into it
What makes me nuts is how you can see the container once you put the arrangement on the cake. It’s hard to remember when the florist is working on the topper that they are working on a table. When the topper goes on the cake, it’s now at eye level and the flowers don’t come down enough to hide the container.
And, since I’m on this soapbox I always tell clients that EVERYONE is looking at the cake. There are only 8 or 10 or 12 people looking at any ONE centerpiece. So I want the good flowers, not the dregs left at the end when all the centerpieces are done.