Not to be Debbie Downer, but I hope you are reasonably certain that she will say yes. This would be a big project that no one would want if she turns you down. (see recent Yahoo news for a story about a fellow who sold his 20-year collection of baseball cards to buy a ring, and his GF said No Way.)
How about having a quilt made with a secret pocket? And the ring could go in the pocket. Then you could ask her. If she says no, you will not be stuck with a "Will You Marry Me?" quilt that will have unhappy memories.