A bucket quarter-filled with water with a towel draped over the side and a fan blowing air over the towel will generate a fair bit of humidity. How effective it is would depend on how big the grow space is and how quickly the air in the space is being vented. Someone on this forum reported that towel-in-bucket-with-fan increased the humidity in their closet setup by 20% or so, though I can't remember who reported that.