You're probably right there; I've seen pictures of him in a slouch hat, but far more in the pith helmet. Redford refused to wear one, saying it looked preposterous. Frankly, nothing could be more preposterous to me than casting a blatantly-American actor as a British colonial aristocrat, although I think he was a great actor in more appropriate roles. It certainly spoiled the movie for me, just like that arrogant turd McQueen totally ruined The Great Escape.

On a related headgear point, all the photos I've ever seen of Bror Blixen show him in a big slouch hat.