At some point I'd like to check out this possibility. Maybe just bend an aluminium plate. I haven't checked if there are good mounting options yet.
I'm surprised no one does this. I don't mean a fully enclosed guard, just a fling blocker.

It would be a lot less effort than cleaning the rim a million times.
I end up thinking "Hmm, I should spray my chain... nah, I've just cleaned it"

or just get some decent chain lube.... I use Wurth dry lube and its never sprays off