I claim the ornamental design for a base of a caulking nozzle, as shown and described.
FIG. 1 is a side view of a base of caulking nozzle, the opposite side view is omitted as it is a mirror image thereof;
FIG. 2 is a perspective view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a front view thereof, the back side view is omitted as it is a mirror image thereof; and,
FIG. 5 is a bottom view thereof.
The broken lines shown on the base of caulking nozzle are for the purpose of illustrating environmental structure and form no part of the claimed design. The design is shown broken away in FIGS. 1, 2, and 4 to show that a particular length of caulking nozzle forms no part of the claimed design.