Corset Dress -- HELP!

Hey y'all!

So, I'm one of the girls working on costumes for my schools Spring play. There's 2 casts, and me and the other girl in charge are having a bit of a problem: One of the lead characters has a specific dress that we want to replicate from the Broadway play. There's a rather large cast (probably about 50-60 people on each cast list) and we wanna cut down on making as many costumes as we can, espically this one! The whole dress is made out of ribbon (though we're just going to find a dress and basically sew the ribbon design on), and becuase of our budget, we don't wanna spend a lot of money on ribbon. The girls who wears the ribbon are both the same height, but there's a difference in the bust area. We decided to try and have a corset in the back (the dress comes with a sort of cape, so the audience isn't going to be seeing the back) so we can tighten it and loosen it, depending on which girl is wearing it.

Does anyone have any ideas on how we could do that?
Would it be the same as making a corset top?

Thanks so much for your time