Those that have a 40" Gen 1 know how easily the AMNPS works in it but It's not easy to get even heat across the grates. Since I got this smoker in May I've been running four temp probes on the four corners of the second grate from the top. The right rear corner was always way hotter than all the other corners and the left corners were significantly cooler. All the corners were out of balance. I could smoke two 1lb. chubs of ground beef for chili in the center third of the second grate and the finished IT would be ten degrees different from the other.
I made a deflector shown in past posts which was better than no deflector then saw Bear's simple design to get heat to the left side of the smoker as well as away from the smoker temp sensor. Then the heat would come back to the right side out the top vent. That was the best heat deflection so far but still big corner temp differences. So I used his idea with a few of mine. The pic below is what I'm doing for even heating.
The water pan sits closer to the door than the back wall because the grate holders aren't centered on the side walls, directing heat up the back wall. If you move the water pan up an inch to the flat grate holder or any other position other than the one intended to hold the water pan, it can be centered in the smoker. Just put a grate over the water pan and have the two parallel wires on the grate straddle the pointed ends of the water pan then lift off the grate put it at the level you want.
Cardboard burns at 451*F and many food companies use cardboard containers that can be heated in the oven as long as it's seven inches away from the heating element. I have 8.5"X11" standard paper sized pieces of cardboard that come one in a package of my business order forms. I put a grate on the bottom Ievel with the beveled edge towards the door so the grate is flat at the right rear corner. I wrapped the piece of cardboard in aluminum foil (shown not foiled for contrast) and butt an 11" side up against the back wall and as far as it could go to the right wall (there's a small crack between the cardboard and right wall where the cardboard butts up to the grate holder.) The cardboard blocks heat from going up the back wall/right rear corner past the MES temp sensor straight to the vent. The foil wrapped cardboard isn't phased from the heat. It's the right size, disposable and available to me. I can always look for something more durable like Bear has.
Moving the water pan to the second from the bottom level allows heat to not be trapped. It seems to make the heat work it's way around the smoker since the water pan is elevated and centered in the smoker. Drippings that fall into the water pan don't vaporize as much, being farther from the heating element. All top three grates can be used.
This set up gets me heating cycles that coast up and down within 10*F. Setting the MES temp at 235*F gets me an actual 235*F average on the four corner probes. The smoker above is a new spare I'll use as an outdoor warming oven for up coming holidays etc. The $273.00 one from Amazon arrived so dented in the back MB sent me a new one. Both operate flawlessly. I did mock ups in my house. The one outside is black and seasoned. I'm still on the fence with selling this tested but unused smoker.