Doesn't have to be complicated and work around curvy HMD shapes and edges.

E.g. a flat sticker that fits an Index front plate, stuff like that.

Fun patterns or maybe goofy VRCat eyes/face for the front or something.

(Example photo from Reddit https://www.reddit.com/r/ValveIndex/comments/e1ovs2/i_did_a_blue_carbon_fiber_vinyl_wrap_on_my_index/)