The ornamental design for a base for a diffuser, as shown and described.
FIG. 1 is a top isometric view of a base for a diffuser of the present invention;
FIG. 2 is an elevational view of a first side of the base of FIG. 1;
FIG. 3 is an elevational view of a second side of the base of FIG. 1;
FIG. 4 is an elevational view of a third side of the base of FIG. 1, wherein the third side is opposite the first side;
FIG. 5 is an elevational view of a fourth side of the base of FIG. 1, wherein the fourth side is opposite the second side;
FIG. 6 is a top plan view of the base of FIG. 1; and,
FIG. 7 is a bottom plan view of the base of FIG. 1.
The subject matter shown in the figures above in dashed lines forms no part of the claimed invention. Further, the bottom view forms no part of the claimed invention.