Are there any aftermarket fenders that only span from the chainstay to the seatstay? I don't trust the rigidity of full-length mudguards beyond the chainstay and this is only to minimize contamination towards my front derailleur. I would prefer it if there's no cutting/bodging involved.
Not a great answer, but I did something like this on my old MTB.
This isn't a good photo because the mudguard broke (for other reasons) and when I repaired it, the linkages no longer move the same way.
However you can see that the original seatpost clamp was used around the seat tube (in yellow)
At the bottom, the tip is wedged into the triangle space formed by the kickstand bracket and the two chainstays. It is not going anywhere.
On the plus side, these mudguards are cheap and common. But they tend to get brittle and snap.
There's plenty of clearance, its a bit harder to see in the first photo so here's a downward shot.
Why did I do this? Back then I had a rear parcel rack fitted, which was lined with a sheet of stiff plastic that extended backward quite a distance. The only part not covered was where the canti brakes are, and the rack was lower enough so this wasn't a problem.