IMO a good rework for the shield would be to set it for difficulty. Let's say that if you play at the early difficulties (Visitor and Pushover) the shield is infinite and indestructible, from Warrior to Subjugator your shield has a limited time span finite HP before breaking down but is still strong enough to take multiple hits/explosions/rockets before breaking down, and in the Maxed difficulty the shield is back to being indestructible but keeps a limited timespan.
As for the timespan itself, for the normal difficulties a good balance would be something that last a minute and a half ( maybe something like 1m20s ) , while in Maxed is reduced to just 40 seconds.