I very much doubt you'll find something of decent quality. Anything out there will be cheaply and quickly made and likely won't fit. I'd just keep an eye on a reputable machinist shop to see when they build one out: https://evotech-performance.com/pages/ducati-multistrada-v4
No disrespect and I don't want to be a seen as a smart ass: Why would anyone want a tail tidy on a bike that's CLEARLY designed to compete with a BMW R1250GS? It's not a Panigale. The new Multi is an all weather ADV bike which will need splash protection. If someone wants a fair weather bike, why would they buy this Multi? Removing splash guards on an ADV bike is akin to adding racing tyres to a bulldozer. It just doesn't make sense. Tell me I'm wrong and please explain the logic because I don't get this....I really don't.
Evotech are in the process of designing and developing a tailtidy for the Multistrada V4. I'll be interested to see what it looks like as the standard rear mudguard / plate holder is hideous. Also wonder if it will work on bikes fitted with radar?
For me, purely aesthetic, I really don't like the standard guard as it sticks out way too much. I understand your argument about all weather, but for me personally, I don't commute on my bike and really only use my 1260 for fun, and as such, I'll try to stick to dry weather, granted, I've been caught out in some horrible rain storms (HORRIBLE....) and even with the shorter tail tidy, I didn't notice any additional splash back. But I did have the top box on. I think it's all down to personal preference. If I was indeed commuting and going on gravel/dirt roads, then I'd likely stick with the original tail.