what is the best humidifier for Ukulele's?

There were several hygrometers (not a humidifier) available when I bought this one. I keep guitars, cellos, ukulele, etc. out (not in cases) in a humidified room. Not trusting any one hygrometer, I bought 4 different ones, among them the Planet Waves. All of them read (and continue to read) within 2% - 3% of each other. However the Planet Waves went through batteries at about (maybe) twice the rate of the other 3, so I stopped using it after a few months. I do not know if this problem is a unit design flaw, or restricted to only the one unit I happened to purchase. I have purchased other Planet Waves equipment and been happy with their stuff. This was my experience. Yours may be different. Again, this is a hygrometer, not a humidifier. If you are really seeking a humidifier, Planet Waves and a host of others make humidifiers for stringed instruments of various designs.

What are the dimensions of this sensor? Can it be adhered to a golf cart with Velcro?

Approximately 3 1/2 inches by 2 1/2 inches. It could be adhered with velcro but being the owner of two of these hygrometers for use in my guitar room, I can attest that they can be up to 8% off. I suggest a humi-care available at cigar stores. It measures 1 1/2 inches in diameter and comes with velcro attachment. Much more accurate and can be calibrated. Not so with Planet Waves. Hope this helps.

D'Addario Accessories Acoustic Guitar Humidifier with Digital Humidity & Temperature sensor, Black

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