Florist use porous wooden stakes (called "Cowee Stakes" after the manufacturer who makes them) to pin two or more pieces of Oasis foam together and build a structure frame.
The porous wood absorbs the water and swells, locking the stake firmly into the wet foam.
Your cone should be very heavy and completely saturated with clean water.
I use a pair of heavy wire snips to cut the stake a little shorter.
I then carefully center the cone over the top of the stake and slide the cone downwards until it is flat on top of the ceramic container.
I will need to insert several more stakes, driven in at angles to assure a solid connection of the cone to the base foam. This ceramic container is pretty heavy, so it can handle the weight of the oversized florist cone.
If you are using a lightweight container, you may want for it to be broader around the bottom so it can balance the weight of the cone without toppling over.