The ornamental design for a fitting for a wall mountable accessory, as shown and described.
FIG. 1 is a left, front perspective view of a fitting for a wall mountable accessory embodying my new design;
FIG. 2 is a left side elevational view thereof, the right side elevational view, the top plan view, and the bottom plan view all being identical to the left side shown; and,
FIG. 3 is a front elevational view thereof.
The broken line representation of a hole in FIGS. 1 and 3 is for the purpose of illustration only, and forms no part of the claimed design.