I have yet to pick up an SKS, it is however next on my wish list. My plan, a 4" PCV pipe a little longer than the rifle without the stock, and a 4" cap. Remove stock from rifle, insert into pcv pipe, and fill with solvent, ie: mineral spirits, kerosene, varsol. Let it soak overnight. Then toothbrush, rags and a full disassembly. I personally will just use my automotive parts washer for everything but the barrel and stock after a 24 hr soak.
For the stock, black garbage bag and hang it out in the sun for a few days. Once the bulk of it is removed, a hairdryer and more rags.
Again, I have yet to actually aquire a SKS and clean rid it of cosmo, but Im confident my plan will work.