The ornamental design for the boxing trainer, as shown and described.
FIG. 1 is a front perspective view showing my new design for a boxing trainer positioned for use on a suspended bag;
FIG. 2 is a perspective view showing the design in an alternate, non-use configuration;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a side elevational view of the left-hand side thereof;
FIG. 5 is a side elevational view of the right-hand side thereof;
FIG. 6 is an inverted top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines showing the suspended bag are included for the purpose of illustrating the environment of the boxing trainer and form no part of the claimed design.