I claim the ornamental design for a hand washing unit attachment, as shown and described.
FIG. 1 shows a perspective view of the hand washing unit attachment from the right hand frontal side.
FIG. 2 shows a front elevation view of the hand washing unit attachment.
FIG. 3 shows a right hand side elevation view of the hand washing unit attachment.
FIG. 4 shows a top view of the hand washing unit attachment.
FIG. 5 shows a left hand side elevation view of the hand washing unit attachment.
FIG. 6 shows a rear elevation view of the hand washing unit attachment; and,
FIG. 7 shows a bottom plan view of the hand washing unit attachment.
The broken line showing of a soap dispenser and roll of towels is included for the purpose of illustrating the hand washing unit attachment in use and forms no part of the claimed design.