For a temporary cover, a tarp of whatever type is the cheapest and easiest. If you attach it across the inside, it won't look that bad.
Next step up would be to board the opening with 2x4 and plywood. If you paint the plywood, it won't be bad. You could even make it into a sign for your business. If you make the board/plywood cover in 2' sections, you can reuse it as shelves, later.
More expensive is a door. Sliding doors made of the same tin as your building and hung on a track on the outside would be a permanent solution.