Can't remember where I saw it but came across a home-made humidifier of sorts. Takes a 2 liter bottle, a drill, some silicone, an aquarium pump and some aquarium pump tubing. You drill a hole in the cap of the bottle and run a section of tubing through it. The free end of the tubing (not in the bottle) goes into your tank to pump moist air into it. The other end (going through the cap) gets siliconed in place.
Then drill a hole about a 1/3 of the way down one side of the bottle. Feed another section of tubing through that and up through the opening of the bottle from which you would pour the contents. Attach an appropriate sized aquarium bubble stone (smaller stone means more bubbles means more humidity output) and then reverse the tubing back down into the interior of the bottle.
Silicone around the hole in the side and give it at least a day or two to completely seal and dry. Fill it with water so it's about an 1 1/2 inches to 2 inches below where you drilled the hole in the side, attach the tube that goes into the sides free end to your aquarium pump, feed the tube coming from the bottle cap into your tank and voila!
Hope that all makes sense...I can't find the link where I originally saw that, but I use it for my BP's and it works great, in conjunction with occasional misting. OBVIOUSLY clean out the interior of the bottle, please! No idea what would happen if you pumped Sprite, Coke, etc. air into there...hate to find out, too! Anyway, hope that helps and good luck!