Cover in Openquest is your main defense against ranged attacks, since you can't dodge them (unless they're thrown weapons). I think with some judicious planning, you can load on enough cover modifiers to make it fairly hard to hit you most of the time (e.g. using a giant shield for a -50% penalty to attackers, combined with standing behind decent cover for another -50%, which will neutralise all but extremely skilled mundane attackers and the magically enhanced). NB: For anyone trying to find the shield cover rules in OQ2, they're not a table and they're not where the text says to find the table, they're on pg. 60 under "Defensive Reactions" subheading "Parries".
Where things start to break down a bit is that many of the ranged attack spells don't have attack rolls. This is the case for several spells where you do get a chance to dodge them, implying to me at least that there is some sort of physical projectile sent on a specific trajectory (several of the spell descriptions also confirm this). So if someone shoots a lightning bolt at you, standing behind a castle wall is useless, because they don't have an attack roll to be penalised.
My proposal is simply that in these situations, cover penalties to attackers' rolls should become bonuses to defenders' dodge rolls. This isn't spelled out in the rules anywhere I could find, but I think it makes sense, and is mechanically simple to resolve. It does mean that huge shields automatically provide a 50% bonus to dodge against spells of this sort, but I'm fine with this, since it makes combat-focused characters (who are more likely to be carrying a shield) more resilient against sorcerers and priests.