5 rules:
12 ipcs
1 hitpoint and can not be captured (just dies if the controller loses that territory) [could also do zero hitpoints, just like AA gun, and still would not die unless territory is lost]
fires a 2 out of 6 before combat begins on all attacking land units, up to a maximum of 10 land units more than the total number of defending units (just like shore bombard, the casualties may fire their casualty shots)
once in place, it is pointing at a certain territory border and only defends against attacks coming from that border (the border can be land or sea, and therefore does not matter if it is amphibious or regular assault)
is built in place, but can only be built in territories that contain an IC or are adjacent to a territory that contains an IC. Only one blockhouse per border.
this i think would be balanced
game should start with only 1 of these units, to represent the maginot line.
the fact that it is built in place, and can be built in a territory that does not have a factory (but must be touching a territory with a factory you control) is a big plus for it, IMO