Most of the shields werent portable as such - they were used as part of the trench fortifications. A prefab firing slit. It was dangerous to observe from the trenches, or to fire from them because of course the other side were trying to do the same thing. So armored shields were one expedient.
Personal armor was another. The Germans even had special arnor plates for helmets and even machineguns.
Another was periscope rifles, I have seen several.
Mobile shields on wheels were tried a bit, but were not suitable to the ground.