I have a rubies and yes, it does need to be larger. Journeyman is right...if your going to do it then do it right.
Get the Don Post and work it. I can't speak for everyone but this seems to be the advise from many here on the board. I'm going to take that same advise myself unless I plan on making my own. Good luck!
B.T.W.----try to stay away from the vinyl/rubber helmets. Most sellers do not include that info and pics aren't always what they seem to be. JMHO