Seeing that there are many kinds of shields, this wouldn't be the only L/E/R- shield in the arsenal.
However, all shield are a pain to wear (speaking from 16 years of experience), it is a given that they all should provide a penalty to AC. However, it would be possible to have one with high BR and only a small penalty to AC (but that would make it unlikely to have any DR). Such a shield would be a Large, Light shield.
An ideal shield for me would have the following stats:
AC -7
BC 16
DR 2
AD -1
HP -3
; That would be a Medium, Heavy shield

There are many ways to compose a shield, but near impossible to make one without penalty, unless it is Small, Light, but then ut would give no significant bonuses statwise. If it were Legendary, it would have some beneficial enchantment to make up for the low stats, though
