Fairings aren't as useful as you might think in most circumstances. On an upright bike, they need to be enormous to offer an appreciable benefit, which makes them heavy and difficult to control in crosswinds.

The optimum solution is a recumbent bike - by going feet-first, you can reduce your frontal area by more than 50%. With a much smaller frontal area and a much lower center of gravity, a tail fairing becomes a practical proposition.

Fully enclosed recumbents can achieve phenomenal results in the right conditions (Sam Whittingham's 91km hour record, Andy Wilkinson's 41 hour LEJoG), but they immediately become a handicap with any sort of gradient because of the ~20kg weight penalty and they're unbearably hot.