I bought a Masterbuilt XL 40, and have tried many different ways to even out the heat, make the wood chunks not burn up in 20 or 30 minutes, and a boat load of other "little" fixes I saw others have done. I tried the cast iron pan; all it did was suck up heat and not radiate it like I had hoped. PLEASE, this is not an attack on what any of the others have done, so if I offend anyone, that was not my intention. Thank God this is America, where we are allowed to voice and vent our issues, without fear of repercussion.
After lots of trial and error, this is what worked for me to keep a nice wisp of smoke going, and not have one end of my rack of ribs dry, and the other end so-so. I removed the factory water pan, and replaced it with a cheapo foil lasagna pan from the dollar store. Yes it had to be crushed a little bit, but it sure holds a good amount of water. As soon as I find the generic brand of lasagna pans at Wallly world, I will modify the factory shelf to accept it. As far as fire/smoke/heat control goes, here is what I did; I had a broken fire bricks and use one half on each side of the burner, I then placed a extra rack I had from a charcoal bbq covering the gas burner. Now I had a way to set my smoke pan (an 8 inch cake tin, another gift from the dollar store) and I can slide it on or off the fire as needed to provide the amount of smoke I wanted. Next, on the water pan rack, I used foil to block the ends from the water pan to the wall, and on the rack above it, I used foil to cover the "front and back", so looking straight down from the top, it appears the only open space is above the water pan What this set up does is eliminate hot spots above the heat source. My vent is set at half way or so, and I never change it. I have found I have way better control of the heat this way. I maintain 225 F at three clicks off off minimum, meaning I use very little propane during the operation of the smoker. My factory thermometer is off about 50F , so I use a digital to really know where I am at. I realize there are many ways to mod your smoker, and please go ahead. All I can say is after about a year of trial and error, I can now walk away from my smoker for hours at a time without worry, and produce some spectacular examples of low and slow.