Hey there so for me, as soon as i introduced a duct fan instead of just clip fans my RH shot way down. The ways i was abke to raise my rh was by watering in the morning so the wet soil would release some moisture as it heated up. Put clear cup/containers over the plants and mist the inside(with a couple vent holes poked in), and then misting the plants and the grow space several times each day. I noticed the rh would be higher at lights out so mostly the day needed a humidity boost. I think you could also put a couple small containers with water in the grow space and let them evaporate out, just change the water often so no bacteria or other build up might happen.
I hope you can find a cheaper solution than using a humidifier, good luck with the experimenting. ✌️