I added a cape to my OSBo by using stackable headers which made modifications to the existing case necessary. In retrospect it might have been easier to just ditch the existing case and come up with a different enclosure. The main function of the case is really just to protect the electronics (IMHO anyway!) and you can protect them with any number of different options out there. I’ve used electrical junction boxes from Home Depot like this one before for a few electronics projects. They’re not expensive, comem in different sizes, weather proof, and I can pick them up locally.