[Suicide Junkie]

Question:
Are the shields a % absorption, or a hitpoints-type dealy?

If the latter, it will be simple.
For the former, a crystalline armor effect would work nicely.
The only issue would be in assuring a fixed percentage absorption...

Of course, if you were to have a set number of impellers, say 5 on each side, then giving each one a CA value of 0.1 would work out.
Damage to the impellers would then cause bigger leaks in the shield effect than the default 50%

You might have an issue with larger ships having more engines and thus more than 50% absorption on the sides, but that may be fine...
The CA adds differently from SE4, so it is a "X shield points generated per point of damage"
Thus, the % leaked is proportional to 1/#CA
A total CA ability of 1.0 gives 50% absorption, so doubling the number of impeller would get you 2.0 CA, and 67% absorption.

Alternatively, you could use custom slots, so that each hull size needs impellers that are specific to it.
A cruiser could require 2x the number of impellers *** a destroyer, but each "Cruiser-sized" impeller would have half the CA ability of a Destroyer-sized one. Thus, each ship would still have 50% absorption even though they have different numbers of impellers. The cruiser's sidewalls would be more resilient, since each point of damage taken would cause a smaller fraction of the impellers to be destroyed.