Heat. Lots and lots of heat. I see you also live in an arid, sandy place. I've had success with leaving everything in a closed metal trash bin (Home Cheapo and Lowers around her still sell 'em in metal). As you mentioned for the trunk trick, wrap it up in rags, set it in the can, close 'er up and set it outside. Preferably in August. Wipe down and replace rags periodically.
I once invested in a
Scünci Steamer, and it kind of works, for spot treatment, but it doesn't put out enough steam to really heat the whole works up enough.
If you have access to a large enough vessel, the "traditional method" (i.e. what they used at the depot in preparation to issue the piece) is to dunk it in boiling water. Eventually, all of the cosmo melts off, and since oil doesn't mix with water, it floats up to the surface, sort of like rendering tallow... If you're equipped for hot salts bluing, the hot water tank would probably work. Or, if you want to shop for it ... Brownell's is your friend there.
In terms of faster than waiting, but cheaper than boiling, you have solvents. Diesel, kerosene, gasoline will all do a nice job. I've had good results with Ed's Red (one of the greatest cleaners of all time; the Googles will give you the recipe, and a few places sell it pre-mixed). Bluntly, though, no "non-toxic", "eco-friendly", "won't melt your face off" cleaners will do much against cosmo.
(Collect milsurps, I do)