It's the same as the aluminum on the outside...
The original was 2024 T3 .032 but you could use 5052. If you are going to polish it, definitely get the 2024 - it polishes better and would match the rest of the panels...but it you want to save some money and are going to paint it, you could go with the 5052.
We mail order aluminum from Airparts
- as near as your computer & delivered to your door. They will cut from the 4' wide roll as much as you need...looks like the going rate right now is $15.75 LF. if you plan on poiishing it, call & talk to them about getting the protective plastic sheeting on the one (good) side...it'll protect it from getting scratched while working on it.