It's time to winterise my hardtail for the oncoming clag and darkness. What's a good rear mud guard if you use a dropper post ... ideally one that doesn't look naff or flop around on your seat rails?
No such thing.
Mudhuggers are fugly but effective
I have a Topeak DeFender M2 which I can just squeeze onto the seat tube below the saddle clamp.
Another vote for Mudhuggers... as said, not pretty but they just work nd decently made and easy to fit.
muckynutz butt fender attaches to the seat rails, so not a problem with dropper post. Its decent enough for preventing skid marks up the back or pack.
I wanted a bigger pukka mud guard so I now have a topeak defender x11, which like the prev poster said can just squeeze the attachment under the main dropper body
Another vote for mudhugger.
Do they still make dirt flaps? I used to have one of them and it kept the dropper post as clean as a butler’s ringpiece.
There hasn't been a decent rear mudguard since the Crud Raceguard from the 90's. They keep your back dry and look smart. Unlike the current ones, that look awful.
If you have 30mm 'clampable' space between the top of your seat clamp, and the bottom of where your dropper flares out (i.e. 30mm of space on the 31.6 diameter bit) then you can pick up old Raceguards on ebay from time to time.
They will only work on hardtails, due to clearance.
I'm sure it was TJ (apols if not you) who years ago (we're talking 15 or so) was big on innovation using a traffic cone. Anyone got a photo? Reminds me of the bodges we used to do way back, prodigious use of old inner tubes for all sorts of frame defending/front mudguardy nonsense.
This is what I use on my full sus.
Take a crudguard and put the Mount underneath and zip tie it on. Cut the superfluous bit of Mount off and you are sorted.
A lot easier to just pop on than a mudhugger.

Sadly it looks like Bentley don’t make the Dirt Flap anymore 😢
Equivalent on eBay though grit guard by 3d reproductions
Ive got one of those ass-savers (milk bottle plastic in the saddle rails) on my CX bike, its surprisingly effective.
For hard tails that rear crus catchers with the the stays was the best.
Crudguard