The ornamental design for finishing material for a showcase, as shown and described.
FIG. 1 is a top and right side perspective view of a finishing material for a showcase showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof, the other side view is mirror image;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof;
FIG. 7 is a bottom and left perspective view thereof;
FIG. 8 is a top and right view thereof shown in environment; and,
FIG. 9 is a front sectional view thereof shown in its position of use, taken along line 9-9 in FIG. 5.
The broken lines showing in FIGS. 8 and 9 are for the purposes of illustrating the environment of the article only and form no part of the claimed design.