Calcium chloride pellets. Brings the RH to about 35% in a closed container. Spread in thin layer, not a couple inches, because it will form into a solid rock instead of liquefying if there's too much. When it absorbs water and liquefies just pour the liquid out and put a new thin layer of pellets in. You can actually dry the liquid back out in an oven if you want to reuse it, 250 F.
You also need to put a small fan in there, like a PC fan, to move the air around or it will take forever to absorb the moisture and mold is more likely. With a fan it comes off smoothly. It can dry stuff right out though. I've had material go down to the low 40s and had to rehydrate it, so you have to watch it and finish the drying more slowly at the end, like with humidity packs at 62% or whatever. Takes about 3-4 days to dry bud completely. It's just a very simple and clean method, all self contained. You can stick the container anywhere you have space and power for the fan. Just mix the material around several times a day for even drying, in a basket or whatever.
