I bought .032" 3003 from Aircraft Spruce:
apparently the 5052 and 3003 are made for the same purposes, w/ 5052 being more "advanced" . . . I think I went w/ the 3003 because I could buy it in longer sheets. Basically you want "workability" . . . both of these alloys are "soft" and tend to hold the shape you give them, vs. something like 2024 which is very springy, resistant to change . . .
I'm pretty happy with the stuff (other than the mental break down I had while trying to install new belly pan at the corners w/shell and frame still "attached" . . . it would have been so much easier as a "shell off" . . . and even better if you can flip the frame over).