first: what are you growing? keeping high humidity is only an issue for neps, heli's, a handful of Drosera and a handfull of Utrics require constant high humidity.
most Drosera, Utrics, VFT's, Sarrs and a handful of neps dont need high humidity. infact most can handlee humidity as low as 20%, ive even had Nepenthes x ventrata pitcher in that low, though most of my other neps are MUCH fussier.
however in a small aquarium type set up in order to keep the humidity up i would leav the top about 75% covered(this depends on where your at, more coverage if your in Pheonix, less on th gulf coast) and use an air pump designed for fish tanks and keep a few inches of water in the bottom of the tank with the air pump going to an air stone sitting in the water. the down side is that it wont work if you have to skip town for more than a day or so as it will evaporate the water fairly quick. or you could run it to a timer to kick on for 10 minuts every hour or something like that. prolly wont need a pump stronger than one designed for 10-15 gallon tanks. now i havent done this setup myself but there is no reason it wont work and have been thinking of using it for Heli's and epiphytic Utrics