A mechanism for enveloping a large round bale with a surface wrap material such as plastic sheet or net includes a housing for holding a supply roll of the wrap material. The mechanism and, hence, the housing is mounted on the rear of a bale discharge gate. The housing includes a front wall, defined by an upright panel secured across the rear of the gate, and a bottom wall sloped upwardly and rearwardly relative to the front wall. The bottom wall and front wall converge toward a wrap material passage defined between a forward edge of the bottom wall and the front wall. The bottom wall is curved convexly upwardly between front and rear ends thereof, with the curvature and slope of the bottom wall being chosen such that a roll of wrap material resting on the bottom wall will become increasingly wedged against the front and bottom walls as the material is used and the weight and diameter of the roll decreases with the result that the braking force imposed on the roll will remain substantially constant no matter what the weight or size of the remaining roll.