Ahead to sides? As in an arc facing from straight forward to straight east? Then place the weapon in the northeast corner of the ship!
edit: oh, yeah, now I understand
Well that would really depend on the hull configuration, so maybe it would be better to have the arc direction determined by the slot's data entry in the ship design... problem is, since in SE5 there are no "weapon" specific slots you'd need to define a firing arc for every slot in the hull!
Maybe weapons should be limited to outer hull, though... that might narrow it down a bit...