I've got the opposite problem, because I'm so short, can't fit a seat post light without the panniers hiding it.
If seat post too short, or the rack bag is hiding the rear light, try ....
- take the red lens and batteries out
- drill 4 small holes through the back of the light
- use 2 small zip ties, through the holes and through the rack bag or if lucky, the bag will have something external for the zip ties to go round.
- keep the excess of the zip ties and these can help to keep the light in the vertical position required.
- when drilling the holes, have to find a location that wont interfere with the batteries when you put them back in.
Keith