Magnesium sulfate should be good; I see that it has an efficiency equal to silica gel, and potentially higher(?). It's rechargable too.

Here, follow this link and open the PDF there... http://www.avantormaterials.com/sear...election guide

This "sweet spot" intrigues me... I wonder if completely dehydrating them and storing them in the fridge (safely sealed of course) would allow you to store them for several weeks. Then when you're ready print, you could take them out and let them acclimate to the humidity level of your house.