Of course, the minimum requirement is a simple agreement to create a marriage between a man and a woman.
I know of at least one woman who had a simple agreement, and got pregnant, and then the man disappeared, so I highly recommend for the woman to have public witnesses to the agreement. That would not prevent a man from running off, but I think it helps.
I think a written covenant is an excellent idea, to help set expectations.
There are a number of weddings in the Bible, so if you can afford it, by all means throw a party!
The closest I ever got personally was a proposal, that went nowhere and then downhill and eventually destroyed the friendship.
What I have done is officiated at several polygamy weddings. These have both been quiet home affairs with family and a few close friends, a short ceremony, and then a modest, but fun celebration with food and drink.
For those wanting more formal legal arrangements you can form a trust, I believe, but see a lawyer for real legal advice.
The best security for her is to pick a genuine godly man, and the best thing you can do for her to provide that security is to model being a faithful follower of Jesus. It is much easier for her to fall in line and be secure if she sees you living up to your profession of faith, instead of saying one thing and doing another. There is far more to being a patriarch than simply having more than one wife.
