You can see the code at https://github.com/kdocki/marionette-twitter-bootstrap-modal-mixin
You can see a demo at http://kdocki.github.io/marionette-twitter-bootstrap-modal-mixin
I could have stuck to single inheritance by extending a Marionette.ItemView or something. But what if you wanted to use a Marionette.CompositeView? You’d end up writing the same class twice, so by using the mixin you are able to ‘mix in’ only the functionality needed for the modal.
After talking to Brian Mann about it, he suggested I use a Marionette.Region to do modals which is how I do modals now. I might do another post on this later.