The ornamental design for a tamper holder, as shown and described.
FIG. 1 is a perspective view of a tamper holder showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left-side view thereof;
FIG. 5 is a right-side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a bottom perspective view thereof.
The broken lines in the figures are included for the purpose of illustrating portions of a tamper holder or its environment and form no part of the claimed design.