FIG. 1 is a perspective view of a sprinkler base according to the invention; FIG. 2 is a front elevational view of the sprinkler base of FIG. 1; FIG. 3 is a top plan view of the sprinkler base of FIG. 1; FIG. 4 is a left side elevational view of the sprinkler base of FIG. 1; FIG. 5 is a right side elevational view of the sprinkler base of FIG. 1; FIG. 6 is a rear elevational view of the sprinkler base of FIG. 1; and, FIG. 7 is a bottom plan view of the sprinkler base of FIG. 1. The broken line showing is for illustrative purposes only and forms no part of the claimed design.