What I did was to tear up a quarter sheet of Bounty (NOT toilet paper!) into 1" squares, feed them in through a barb hole.

Add 1" (measured along the long edge of the res) of distilled water, plug all holes with fingers etc and shake like hell for a while.

The bounty acts like little scrubbers as they are washed around by the (distilled) water inside the res.

Then flush out and remove Bounty using tweezers through a barb hole or filler cap - whichever is easier.

Voila, one clean MCRES.

It won't get rid of 100%, but I'd say it cleans to 90% of new in a few minutes. I got bored at that point and rebuilt my loop muttering "good enough".

HTH
J