I have the same problem, a "streamlined" head so that most helmets end up rocking side to side. You could stick Powerbars between the sides of my head and the sides of the helmet, just about!
Bell is nearly the only helmet that truly fits my head, and I've worn Bell since the first white hard-shells came out in the 70's/80's. I did have one Specialized "Mountain Man" helmet for a while, but then went back to Bell.
Right now I have a Bell Citi, with a nice rounded shape (no faux aerodynamic spike in the rear) and in a bright visible yellow. Great helmet, nice fit for my head, and not too expensive.
I wrote a review of it somewhere...
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson