If covering the screen still doesn't do enough, or you can't cover enough of it because of lamps or whatever, you need to add more water to the cage in some form that will let it evaporate over the course of the day. Misting doesn't work for more than a little while for two reason: All the moisture introduced that way is in small droplets all over the place, and it all evaporates at once causing a spike in humidity that then escapes the cage with nothing left to keep evaporating; and it's actually just not that much total water anyway. Picture watering a plant with a spray bottle. If it's a large-ish plant, you're going to be pumping that handle for a *long* time.

But you can add a larger water bowl, positioned over/under a heat source for more evaporation. Or you can add other things that can hold a lot of moisture and release it slowly, such as a large bowl full of damp moss. For that matter, live plants are great because the soil holds a lot of moisture and releases it slowly, and you replace it when you water the plant. And the leaves release moisture too. Something like spider plant or pothos vines is safe for your snake and will provide the snake with some cover, and they're sturdy enough to not die from being sat on.

You can also make a humid hide, that you keep a handful of damp moss in.