This is a picture of a cello Dampit with two ex-asparagus rubber bands on its upper end. It had been sitting on my bookshelf for years, ever since Ben and I fished it out of his cello years and years ago (it somehow made its way into the body of the instrument, and never went in there again).
Suddenly I got the idea that it might make a good humidification system for my violin, so I removed the string tube (I never used it anyway), and replaced it with this Dampit.
As you can see I have a stretto device attached to my case. Once you let the little bags that go in the plastic case dry up completely, they never work again. Sometimes they get moldy, and then they lose their ability to absorb water. It is easy to forget to saturate a stretto gel bag when you can't actually see if it needs saturation. After going through a couple of gel bags (two come with the system), I ended up putting a bit of sponge in mine, which does very little to help humidify my instrument. The Dampit does a lot.
The cello dampit goes for ten bucks. A Stretto with two gel bags costs $30.00, and a replacement gel bag costs $11.00 (plus postage).