I don't want to get married without a prenup
Why do you need a prenup to get married?
I have one special material possession I want to keep
A lot of people would disagree with you and say that if you're already planning for your relationship to fall apart that you shouldn't get married just yet. You should speak with the potential fiancee and see how they feel about a prenup first.
I'm just being prepared.
My only advice would be to just spend a lot of time with them and see if you can trust them without a prenup. Most people would think it is weird to be obsessing over the potential end of your relationship so soon.