We claim the ornamental design for a smart device holder, as shown and described.
FIG. 1 is a top, front, right side perspective view of a smart device holder showing our new design;
FIG. 2 is a top, rear, right side perspective view thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is a right side elevation view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken lines shown in the Figures represent portions of the smart device holder that form no part of the claimed design. The short dash-long dash broken lines shown in FIGS. 1 and 3 define the bounds of the claimed design and form no part thereof.