Assuming you got the AMNPS 5x8 which is made for the MES30, the smoker goes on the two rails just to the left of the heating unit. It is made to fit like a glove and you get good ventilation from below with the tray suspended on the rails. I leave my chip loader tray open about 2" to allow air flow to keep the AMNPS to burn consistently.
I've struggled with keeping my AMZN lit. I left the top vents open and layed the tray on the rods to the left of the chip tray - nothing else. Not sure what your "mailbox" mod is. Are you saying pull the exterior chip tray out 2" for vent, or the interior tray? Thanks!