I like the Oasis humidifiers. I would buy the case humidifier one as I
find that's the most convenient to use. It did not work well with one
Eastman plastic formed case, but I found that a CD works well as a
bracket to suspend the Oasis humidifier from the (flattop guitar)
strings into the sound hole. The hole in a CD is the right size for
the top of the oasis to feed through and the cap will hold the CD in
place just like an Oasis plastic sound hole style bracket. Also note
that the Oasis comes in two different permeability grades depending on
the weather conditions. (There's a blue and a tan colored model with
the tan for the drier weather.) I have some of each and like the blue
only for a good fitting case where the guitar spends all of it's time in
the case.
Dampits work and I use them in one Eastman case with an archtop. I
think the case does not close tightly as they dry out quickly. I
actually use two of them in the one case. I hook them on the strings
below the bridge and feed them into the one F hole, (noting rubber rests
on the plastic ivoroid). I find these to be a bit of a pain to use as
you have to dampen them, squeeze out the a little water, and dry the
outside off before putting them back in the guitar.
I still recall seeing a luthier friend just using an old plastic soap
box with holes drilled into it with a good size damp sponge inside the box.
Of course I think nothing beats having a small room where you can easily
maintain and monitor the humidity levels in the room.
Danielle