If I had that smoker I would fill the water pan with dried beans like you do for filling a pie crust and pre baking it. I would fill it with dried beans and cover it with 2 layers of heavy aluminum foil. I would put my tray right on top of that water pan or the rack above it. I would cover the tray with a piece of aluminum foil drip pan bent to keep a 1 inch gap above the top of the tray and pellets. Essentially making a vented cover for the tray allowing smoke to roll out of the ends and or sides. You give up the bottom rack cooking area when you choose to use the tray .
Or..... You buy the tube thingy.. :D
I don't know what the beans are supposed to do, but setting the AMNPS right on top of a non-porous thing like a foil covered pile of beans wouldn't allow enough air flow through the AMNPS.
Putting it on the bottom rack is good, because air can get flowing through it, and you aren't losing much of the bottom rack, besides the bottom rack is too close to the heat for smoking meat anyway.
And putting a foil pan cover only an inch above the AMNPS will partially smoother the AMNPS by blocking air flow. Any Cover must be farther above it.
Bear