Thanks to all the great hints, I have done most of the "standard" mods--including building a charcoal basket. I tried to extend the vent with a semi-rigid aluminum duct, but found that 3" was just too small and 4" was just too big. I could not find one that was just right (the "Goldilocks problem"). I even tried 4" duct with a 3" clamp, and the duct kept unraveling. I ended up using aluminum flashing, but was initially bummed at losing my warmer rack. Solved that problem by "modifying" the rack so that the vent goes through it (see pictures).
Next up: address the heat loss from the side and back of the main grill and baffle. Thus far, I've smokes (hickory and pecan) a few Boston butts and they came out great!