My goodness, there are a lot of recipes for this out there. Some are simple and cheap and easy, but really don't do humidity very well. Some are too expensive and time consuming to even attempt. Rather just go buy the stuff.
After several attempts and failures, using others recipes, I gave in, and modified a tradition paper mâché paste. Works beautifully.
1 cup con starch
1/4 cup sugar
1tsp pickling salt
4 quarts water
Sift the dry ingredients together. Slowly stir in water, using a whisk. Make sure to get all the lumps out. Grab a book and a chair. Sit in front of your stove. In a large pot, over a medium heat, constantly stir the liquid until it becomes translucent and thickens. This can take 40-60 mins. Be sure it's a good book. Cool slightly, pour into a storage container. Be sure to wash out your pot.
Can be used as soon as it gets to room temperature. Stores for several weeks, however if it dries up, slowly add a bit of hot water, whisking, until it is usable once again.