Divx works for the most parts but can be a bit buggy at times.
Ive moved over to just LavFilters + Icaros Thumbnailer.
For me, I need Haali (and prefer it) on the 32bit side of things with FFDshow.
So, what I do is install just the x64 LAV Splitter and LAV Video, and set it for MKVs.
Then Install and activate Icaros.
This takes care of MKV thumbnailing nicely in Windows explorer, as well as playback in WMC and the 64bit players.
Unless you have some oddball video/audio streams.
I sill use FFDShow and haali on the 32bit side though, because I prefer it for AVIsynth and video encoding.. but if you do not have a special need for it, you can use LAV splitter/video on the 32bit side as well.
One other thing you may try, if you are happy with DivX, is clearing your thumbnail cache.
Either in disc cleanup or with CCleaner.
Then reboot and see if that forces DivX to make the thumbs properly again. it should since it will force new one to be generated.