I think the absolute latest UA flight is FRA-SFO (5:25pm), which doesn't help the OP I guess. Other non-UA flights to consider in this case are Privatair/Swiss ZRH-EWR (4:55pm), ZRH-BOS (5:30pm) or Air Canada LHR-YYZ (7pm)

I often get frustrated by this problem of such few late afternoon westbound TATL flights.