The high speed rotary cutter bed of a harvester or other mowing machine has a rubberized deflector curtain that wraps around opposite ends and front extremities of the cutter bed to guard against thrown objects, yet permit the ingress of standing crop materials as the machine is advanced. The curtain is divided into a series of discrete sections which mutually overlap along their upright marginal edges so as to permit independent raising and lowering of the curtain sections with their respective, overhead rigid covers into positions permitting access to the cutter bed for maintenance or other purposes. Self-actuating retainer means in the form of magnetically attracted components on the marginal overlapping portions of the curtain sections cause the sections to be automatically readhered together into an effective single deflector curtain around the cutter bed when the curtains are down in their operating positions, yet allow the curtain sections to automatically release from one another when their respective cover sections are raised to their access positions.