I did that to use with this mod: http://skyrim.nexusmods.com/mods/7898 - Ethereal Shields beta
The author used the glass shield model to make a shield without armor rating, so mages could use one and still keep the mage armor bonus.
Thing is, the Glass Shield, as nice as it looks, still looks bulky for a mage, so I remembered "what about the ghostly weapons? I'm sure there is a ghostly shield, since I didn't explore every corner of the game yet!"
And there wasn't. There wasn't even an "ancient nordic shield". So, I made this and changed the directories in the esp, pointing to the right files. :p
Just a side-note: You can use it as a replacement, since it uses vanilla meshes and textures. Just put it in the folder of the shield you want to replace and rename correctly, like "Data\meshes\armor\shieldofysgramor\shieldofysgramor.nif" [mine is called ghostlyshieldofysgramor.nif]
Edit: The whole purpose of this mod got surpassed by two things. One, the fact that using a shield doesn't count as wearing armor, thus not affecting the mage armor perk. And two, the mod Staves of Skyrim [ http://skyrim.nexusmods.com/mods/22691 ] is a MUCH BETTER solution to the whole mage and shield problem. Check it before you make any assumptions!