Maybe her name in her own writing.. do you still have any cards from her in which she's signed her name but not a proper signature? Just 'Donna' as she used to write it would be beautiful. I read of a guy whose father was dying and he had his father write out a phrase he used to say to the son as a child.. now he has it tattooed upon him in his father's writing, it's a beautiful idea.
I'm sorry for your loss, and it's lovely that you want a part of her upon you always, then she'll always be with you both in your memories and in your heart as well as a physical part of you too.