Posting this here, too.
I just posted in a thread called "The Cake Bible!!!". Once I looked at the original post from 2010, it wasn't even about MC, of course, but had gotten off track talking about a cake recipe. Which somehow turned to humidity and modeling chocolate.
Anyway, I am sure many have seen this, but thought it would be helpful for those that didn't.
Kaysie Lackey via FB: Recipe for high humidity (this was for a class she taught in Malaysia and she was worried about the behavior of the modeling chocolate). It was also noted in the end of the post that it even worked without air conditioning.
Hey Guys! The recipe is 1000g white chocolate (23-28%) to 500g glucose, plus 2 tsp of water. The water amount depends on your glucose (in Penang it was really REALLY thick, so it was 2 T instead of tsp). You must melt the chocolate in a double boiler, and in the very warm glucose mixture, and stir to combine completely over the heat (sorry... no microwaves). Turn it out on a lined sheet pan and let set overnight. This is based on Richard Festen's method of making modeling chocolate that you can find on his Chocolate Stripes DVD, but tweeked by me to work in countries where candy melts and corn syrup are hard to find.
Further questions and answers followed the whole things, so if you need more info, I think you can just click on Kaysie Lackey's name up there and it will take you where you want to be.