I think 1.90" in length and the post seems to be 0.40" in diameter, so the sleeve would have to allow for that size of post. I leave my headrests in the same position. I think the head rest posts have a curve in them, I tried to straighten my posts because the headrest was leaning forward to much.
As for the design, I'm thinking a bevel on both ends similar to the door pin sleeve you sell with your door pins.
Total of 4 sleeves to do the passenger and driver seats.