The ornamental design for a mountingfixture for a roller blind box, as shown as shown and described.
FIG. 1 is a perspective view, taken from the top, rear, and left side, of a mounting fixture for a roller blind box, in accordance with our new design;
FIG. 2 is a perspective view thereof, taken from the bottom, front, and right side;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is a right side elevation view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The break lines shown in FIGS. 1-4, 7, and 8 symbolize breaks in the length of the article in which the claimed design is embodied. The appearance of any portion of the article between the break lines forms no part of the claimed design.