The Pilgrims lost 7-1 at RAF College Cranwell, having trailed 2-0 at half-time to the physically-superior Harriers.
Bradley Easey scored United’s solitary goal — but the Pilgrims were already 5-0 behind at that stage.
Boston (3-5-2): Jamie Bradley; Billy Hodgkiss, Daniel Moulds, Joe Cain; Daniel Worthington, Miles Lynch, Corey Newman, Liam Moran (Jack Foyster 55), Jack McCallum (Conor Allen 52); Louis Saebe (Scott Floyd 70), Bradley Easey.