Ukraine

https://www.mk.ru/economics/2024/11/27/cb-yavno-opozdal-s-merami-podderzhki-rublya.html
He does mention one item (if this doesn't work) having Russian exporters sell off part of their foreign currency earnings. Not sure how that helps (I'll read up on later) but they halted that earlier in year.
It was 80% within six months and 90% within few weeks after that. That created an artificial demand for the ruble, which propped up its value.

The downside is businesses would be less inclined to do business in Russia as it is a pretty heavy handed regulation. But many of the Western based businesses have already left anyway. The ones that remain are heavily betting on Russia being able to weather this out.