Fancy option under seat – http://four4th.co.uk/products/lights/scorpion/
I have one of these though and just wrap it round the small gap I have between Reverb collar and top of seat tube…
http://road.cc/content/review/108100-moon-crescent-rear-light
Obviously need to not be running dropper right down to collar, or maybe it’ll fit the seat tube but the angle needs to be right to be seen and not blocked by tyre.
I just use it for longer linking road sections between trails or a get me home on road situation. Has a rubber strap that’s easy to quick fit so can keep it in a pocket or backpack as it’s fairly small, or you can leave the strap on and there’s a plastic quick release for the light but I find it more hassle.
If just using temporarily you could strap round the dropper post itself assuming you’re not going to be dropping the post while on the road.