(FT) immobile targets vs salvo missiles

Anyone have any good ideas to make immobile units, such as a 
base, less vulnerable to salvo missiles?  That is, without a full 
fleet in attendance, lots of escorting fighters, or nearly its
entire weapon outlay devoted to PDS, since all of this could
be countered by the enemy having more missiles and/or a
bigger fleet?  One idea I thought of, only allowed on bases of 
x+ sizes, is a jammer-network where only half of the missiles
that get a target lock then lose than lock.  Example: 1 salvo,
player rolls a 5, so 5 missiles lock onto the target, is then
halved to 3 missiles that actually have to be defended against.

Maybe drives interfere with the jammer-network (and vice-versa) 
which is why ships do not have them (PSB reason why some 
missiles miss).  Or, has a large mass outlay (say 20%?) which 
makes it uneconomical for ships.

For now, our new campaign does not have to worry about
Nova Cannons or Wave Guns since we're using book ships
(have I said how I hate this idea?  at least I'm allowed to
replace the single hangar bays from any DNs that I buy).

Glen