The shield should be in the armoury, in the south west part of the castle, on a bench in the south west part of the room. The key for the armoury is in a room to the north of there.
Since the shield has random stats, it would morph if you tried to swap inventory files to get it. However, since it is on the ground, you may be able to follow the same general procedure, but backup and swap the world.x0 file from a save before you entered the castle. This file keeps track of things on the ground, as well as quest opened doors and hatches. I have not tried this, so am not sure if it would work properly.