The 24 Heures du Mans has only once been won by a driver in a car of his own making as far as I know. Le Mans resident Jean Rondeau won in this car bearing his own name in 1980 – the Rondeau-Cosworth M379B – Check out the Wiki page for more info.
Photo taken at Goodwood Festival of Speed earlier this year, for more photos from FOS check out this category.