A vehicle door could be used as an improvised shield or crafted into a dedicated shield. When held, it would protect the upper and mid-body from gunfire. To keep it balanced, players should only be able to use melee weapons or, at most, a pistol while holding it.
The shield should also have limited durability and only block a certain number of hits before being destroyed. It should also slow the player down, keeping it as a tactical defensive option rather than making it too powerful.