The ornamental design for a monolithicductspacer, as shown and described.
FIG. 1 is a front perspective view of my new design for a monolithic duct spacer;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a rear elevational view thereof;
FIG. 6 is bottom plan view thereof; and,
FIG. 7 is a left side view thereof.
The broken lines are included for the purpose of illustrating portions of the monolithic duct spacer and form no part of the claimed design. The dot dash dot lines immediately adjacent to the shaded areas represent bounds of the claimed design.