Here are the actual reprinted rules from AA D-Day. Use this and come up with something based on the designers intent.
Blockhouses
Only the Axis has block houses.
Block houses do not move. They fire
in combat against land units in their
zone just as other land units do,
attacking on a 3 and defending on a 1.
Blockhouses also can make a special
targeted attack out to sea, directed
by order card 9. Each blockhouse
may fire once at an Allies land unit
in a beachhead box within its firing
arc. For example, the two
blockhouses in the zone containing
Bayeux can each fire on land units in
the Gold Beach box. (Not all blockhouses
are in a zone with an adjacent
beach head box.) A roll of 3 or less is
a hit. Each block house can fire on a
different target.
Naval Bombardment
The Allies can conduct a naval
bombardment when an order card so
directs. Naval bombardment targets
only blockhouses. Roll six dice, one
at a time : A roll of 2 or less destroys
the blockhouse.
The Allies choose a block house as
the target of each die roll; the same
block house can be chosen more than
once.