RM2 variant stuff warning
I considered allowing the number of facings covered, on say a hex grid, based on the shield size. Target shield could cover 1 hex facing a round, Normal - 2, Full - 3 and Wall - 4. Generically arbitrary yes, but it had a nice feel to it.
We also allowed shield parry/coverage vs multiple foes if within the correct arc -and- you had OB enough to spread about.
YMMV,
Old Man