I think a 4 to 5 star helmet with normal and elemental defence that does extra to costructs would be good. It would work well with the silvermail armor the same way the angelic armor/helm's and drake armor/helm's work togeather.
This is becuase it would block against all forms of damage to a considerable amount and it would (with silver mail) provide some form of bonus against monsters weak to one element (undead and constructs are weak to elemental), allowing you to use your two slots for a peircing and shadow weapon to be well rounded.
For example, drake and angelic mixes provide bonuses to beasts and feinds, whom are both weak to peircing, allowing me to use a shadow and elemental weapon, and have defense against normal, shadow, peircing, and elemental damage. Plus nice bonuses against curse and posin.