That is a wonderful solution!
I have a wood vice, but my roommate has a metal vice. He has these magnetic covers just like you made, but his are hard plastic. They are pretty effective at protecting the wood, but he always get grease on them, so I only use my wood vice.
The guitar is going to be sweet!
i love working on cars too, but If I start doing that... I think I need another shop (lol). Can't mix sawdust and grease!