You could go either way. Make sure you wrap it in a couple layers of foil. Honestly what is do is fill the water pan with sand, cover that with foil. You get the same results as using the clay planter tray.
I'd sit it in the pan. If it's just supported around the edges it'll break sooner than if it's supported on the bottom too. It's going to break eventually no matter what, but at least if it's sitting in the pan it won't fall into the coals and get ashes all over your food.
That's a point I hadn't thought about Boat. I elected to put the saucer on the pan. I just felt it was more stable. I used it today for the first time and it worked out well. Thanks so much for your insight.