Adjustable brush holder

FIG. 1 is a top view of my new, original and ornamental design for an adjustable brush holder;

FIG. 2 is a bottom view thereof;

FIG. 3 is a front view thereof;

FIG. 4 is a rear view thereof;

FIG. 5 is a right side view thereof;

FIG. 6 is a reduced top left perspective view thereof; and,

FIG. 7 is a reduced top front perspective view thereof.

The broken lines showing a paintbrush, screws, and a container illustrate environmental structure which forms no part of the claimed design.