We claim the ornamental design for a video game controller thumbstick attachment, as shown and described.
FIG. 1 is a top perspective view of a video game controller thumbstick attachment showing the new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is a first side elevation view thereof;
FIG. 5 is a second side elevation view thereof; and,
FIG. 6 is a top perspective view thereof showing the video game controller thumbstick attachment positioned on a thumbstick of a video game controller.
The broken lines in the drawings depict environmental structure of the video game controller thumbstick attachment that form no part of the claimed design.