The ornamental design for a heaterbox, as shown and described.
FIG. 1 is a front and top perspective view of a heater box, showing my new design;
FIG. 2 is a rear and top perspective view thereof;
FIG. 3 is a front end view thereof;
FIG. 4 is a rear end view thereof;
FIG. 5 is a left side view thereof, the right side view being a mirror image;
FIG. 6 is a top plan view thereof, the bottom plan view being a mirror image; and,
FIG. 7 is a perspective view thereof shown in a used condition with a hunting stand.
The broken lines in FIGS. 1-6 represent portions of the article that form no part of the claimed design. The dot-dash-dash lines in FIG. 7 represent environmental structure that forms no part of the claimed design.