We claim the ornamental design for a 3D printer accessory, as shown and described.
FIG. 1 is a right perspective view of a 3D printer accessory;
FIG. 2 is left perspective view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a rear view thereof;
FIG. 6 is a front view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
The broken lines immediately adjacent to the claim that depict unshaded regions depict the bounds of the claimed design, while all other broken lines are directed to environment. The broken lines form no part of the claimed design.