Maybe it contained his lunch and laundry. After all he regularly collected his laundry after visiting Marina. Yet Buell only remembered Oswald carrying his lunch.....a dual purpose bag....there's an option.
Well, it's lucky for the conspirators that their patsy, on the day of the events, just happened to decide to place his lunch inside a twenty-four inch long bag that contained his laundry. This fit perfectly with their need to have it appear that the patsy carried his rifle into the building.
I'd rather be lucky than good.