My helmet is made of thin craft foam and it fits just fine.. its also snug against my head like it should be.
Who's model did you use? If you're doin foam then you shouldn't have to take the bottom part out in order to fit you head in.
Try this one! I think it is the most accurate Iron Man helmet out there. It doesn't have those huge bulky edges on the bottom that block your head from sliding in like robo cat's does.
01 Helmet(Thin craft foam).pdo