The dressing up won't be the problem. Find some baggy chino pants, a worn oxford cloth shirt, a sloppy tie, a tweed jacket, a battered fedora, and a big brown macintosh coat (the earthy Lewis referred to it as "dung-colored".) What'll be hard to duplicate would be the mannerisms. He liked to laugh long and loud and he could be abrupt and combative in conversation, yet could turn right around and be as considerate and gentle a man as you'd ever find. If you want a good description of what Lewis the person was like, look up a copy of Sheldon Vanauken's A Severe Mercy and read the chapters following "Encounter With Light".