Personally I would look into getting a Motocross one as the MTB specific ones have no special safety standards over an xc lid.
Which in turn is no more special than a roadie lid, which is no more special than a £5.99 one from woolworths.
A full face helmet form a reputable brand is going to be a lot more protective than a XC lid. If not quite upto MX standards, but then we're not going 60mph with 200kg lumps of metal flying about the place in a crash.
661 do a couple of different head shapes, one (the strike IIRC) fits me like a glove (better infact than my motorbike helmet) the other wouldn't even go over my head. Was arround £35 from CRC, reduced from £80ish. The weight isnt noticable as its COG is in th emiddle of your head rather than perched on top like an XC lid.