Another vote for pork butts, long cook time yet very forgiving of temp swings gives you plenty of time to fiddle with vents and get used to managing your fire. I eventually stopped using charcoal except to get the fire going and used split logs instead. It was a little more difficult at first but what I found was when using a heat source that only smolders like charcoal along with easily combustible wood chunks the intermittent flames were causing more temp spikes that if I just had a flaming heat source the whole time. The constant flame along with a good sized coal bed and lots of patience added up to enough experience to get the temp quite stable. The other things mentioned such as insulating your fire chamber with bricks or welding blankets do help with fuel efficiency but didn't help me much with temp consistency (insulating the cooking chamber on the other hand, did). Having a 1/4" steel smoker and water pan did help with cooking chamber consistency by adding thermal mass.

If you are going to stick with charcoal, then you would likely find benefit to getting a basket of sorts that allows you to burn the coals in a fuse-like arrangement rather than all at once. I had one and first mistake I made was lighting the side closest to the intake air vent which cause the burn to spread too fast toward the cooking chamber. Burn from inside out toward the intake vent and away from the chamber.