That helps a bit but also,don't be afraid to cover more than just the air vents, the wind and especially rain can absolutely kill a roast, sucking the heat off the grill in a jiffy. They make insulating blankets you can throw over the grill as well or try to insulate it. Now of course you don't want to use fiberglass or anything that is going to get awful on you but even an old blanket, minding you don't get it too hot and start it on fire, just anything to cut the direct contact of the wind with the grill surface can greatly help with heat retention.
I have found, wayyy back in my days where I did the redneck roaster from a coffee can, and an old converted foreman grill, that they have wool blankets you can get sometimes real cheap at the old army / navy surplus stores, drape one of these over the grill can help with heat retention too and is fairly cheap and being wool, will take quite a bit before you did catch it on fire IF you let it get that far.
Speaking of air vents, turning the grill so the vent is down wind of a prevailing breeze can help a bit too, you'll have to play around with your setup and see what specifically works for you. Id recommend getting 5 lbs of lower Q beans and just playing with them, paying more attention to heat rather than roasting properly, so you can get an idea what you need to do for when you put good beans in.
Remember, thermal mass is a godsent. THe more weight you can get heated up, the more stable the temperature will be.
Aaron
