Congrats on your soon-to-be acquisition!
Guild's are well-known for the build quality, and that 12-string should have a big bright voice with the jumbo body, spruce top, and maple sides. I agree with the others, that the best case you can afford for it is the right one. Getting one with a jumbo body AND the extra length to accommodate the longer headstock is the challenge, but gator Cases is a good place to start. If the specs on Guild's site and Gator's site are exact, this one should fit:
Sweetwater and Musician's Friend are both listed as retailers for them, and these molded babies usually cost a couple hundred bucks. But a gorgeous guitar like that - especially since you'll be travelling with it - deserves to be well protected.