We claim the ornamental design for a 3D printer build unit, as shown and described.
FIG. 1 is a front perspective view of a 3D printer build unit, showing our new design;
FIG. 2 is rear perspective view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a front elevation view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is a right side elevation view thereof;
FIG. 7 is an enlarged top plan view thereof; and,
FIG. 8 is an enlarged bottom plan view thereof.
The broken lines immediately adjacent to the shaded areas 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.