The ornamental design of a lightweightportableheatingdevice, showing our new design, as shown and described.
FIG. 1 is a top, front, and right side perspective view of a lightweight portable heating device showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a left view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken line shown in the drawings are for the purpose of illustrating portions of the lightweight portable heating device and form no part of the claimed design.