I've seen some people take traditional PB masks and cut them down. They remove the sections covering the cheek area and leave a small section in front that covers the mouth. Having that extra material removed from your cheeks should allow for use of iron sights.
You could also go with one of the Sykes masks that ILLusion sells. I have one but have yet to use it in a game. Reasonably comfortable solution, but prolonged use may be a different story. Jugglez (Airsoftparts.ca) sells a similar type of facemask as well. Check them out.