You'd be suprised at how much water the clay holds, and they don't dry out as fast as packed cotton. I have also used clay kitty litter in film canisters as well, that hold water very well too. But, I agree these might be small. I also have some I made using 35mm film canisters, those work great. I pack them with cotton balls and have been very successful. I buy these little disks and aluminum disk holders from a pipe store because I smoke a pipe and use them in my tobacco cans. That is what these are for, to place in humidors for cigars, and tabacco canisters. I wanted to make one that hangs from the strings inside the body of the uke, and may be good size for that, because you are humidifiying the body from within, and lue of the whole case. That was my thought. I wouldn't use one of these to store a uke for long term, I imagine you would have to dunk these in water to fill them up at least once a month. This one has stayed moists for at least a month, but I had the disc in a stack of them placed in a 35mm canister. It might not stay moist as long on it's own in the little metal can. But, I want to try it. I think I need to pick up a small meter for a cigar humidor to monitor it though.