Aaron,
See this thread for a post of mine.
I can measure the thickness of the padding Rob puts in if you like, but really you can put any sort of padding you want. His helmets are scaled up a bit in size to be in proportion to the RT-MOD suit not scaled up for a larger head.
After having personally fondling an both an AP and TE2 in comparison to my RT-MOD, I firmly believe that any helmet will fit a big giant melon with modifications.
Aloha,
-Eric