Humidity is just water content in the air right ? So just get large electric misters and spray by hand up to 6 times a day ... Other than that you need to increase the humidity of your intake by having more evaporation happen to the air you're inputting .... At the seedling stage like this though it's much easier than that : just use domes on top of the seedlings : inside the dome it should hit the 70s and 80s like you want it to