DIGITEN Humidity Controller Pre-Wired Digital Humidistat with Temperature Display, Dual-Stage Outlet Humidifier /Dehumidifier for Mushroom,Reptile, Greenhouse, Fan, Grow Tent

Searching for your solution I found these options - misters - https://www.amazon.com/Reptile-Humidifier-Terrariums-Automatic-Adjustable/dp/B09KX9YFWW/ https://www.amazon.com/Humidifiers-Terrariums-Humidifier-360%C2%B0Adjustable-Rainforest/dp/B09WXZPP81 humidity controllers - https://www.amazon.com/Humidity-Controller-Inkbird-Humidistat-Pre-wired/dp/B01J1E5LWM/ https://www.amazon.com/DIGITEN-Controller-Humidistat-Temperature-Dehumidifier/dp/B0DLK14Y23 reptile foggers - largest capacity i've seen is 4L but they're open top so you would have to deal with evaporation. I don't think you'll get 3 weeks out of this option. If you want to go crazy DIY you could build a setup with a float switch for an "ATO" or automatic top off commonly used with aquariums and a larger reservoir to top off a reptile fogger. I would only go this route if you have adequate time for testing. Herpetology and aquarium goods will probably be the areas you want to look in, though the issue of course will be the length of time you are gone.