This is a really easy system and works well. It's easy to re-position the pot later too, if needed.
Here's what you will need:
Creative Industries plastic square bat (about $7)
Vinyl screen molding (Home Depot - $6 for 12 feet, so there is plenty to share with friends)
PVC Cement (It's about $2 and also is plenty to share with friends)
Several 6" ceramic floor tiles. They actually measure about 5 3/4", (Mine were about 59 cents each at Lowe's) Buy lots of tiles at the same time, so you can be sure they are all the same size.
Cut the screen molding with the pruning shears. I cut two pieces at 4" and two at 3 1/4".
Place the tile on the bat and position it with the molding pieces so it is in the center. I just eye-balled it.
Glue the molding pieces down using the pvc cement, so the molding is snug up against the tile.
Try not to get the cement under the tile.
Let it dry.
To use, just install the bat on the bat pins, and insert a tile. Throw the pot on the tile and remove it. If it doesn't come up easily you can use a putty knife under one of the exposed corners. Put in a new tile, repeat.
I did this several years ago with concrete pavers and they really chewed up my pinky finger from centering. Glazed tiles are much better.