You do not need industry expertise to build a good initial case structure. The safest starting point is the business question itself: what decision is the client trying to make, and what must be true for that decision to work?
From there, use dimensions that are relevant regardless of industry: customer demand, economics, capabilities, competition, risks, and implementation. You may not need all of them, but they give you a logical starting menu.
The important thing is to avoid filling the structure with fake industry knowledge. If you are unsure how a market works, say what you would want to understand rather than inventing facts. Interviewers are testing whether you can reason through unfamiliar situations, not whether you have memorized every sector.
Once the interviewer gives you more information, refine the structure. A strong case approach should become more specific as you learn, not start perfectly specific before you know anything. A useful reference is case structure vs case framework.