I'm having severe humidity issues

As i stated in the title i am having trouble maintaining humidity. My male killer bee doesn't seem to care much but im worried over time this could hurt him. I mist probably 3-4 times a day and it always drops... STATS FOR NERDS I honestly don't know where to start. When i wake up it's low... mid 20's low 30's. I mist it and it goes up to about 50 :D for about 10 minutes :( then after an hour is down to ~30 again. I can't afford moss from the pet store and sphagnum moss solutions i tried online don't do a good job and break up in a week. Probably not worth 9 bucks for something that ultimately doesn't work. He weighs 230 grams and is about 6-8 months old. Very good eater; you wouldn't even be able to tell im terrible with husbandry. I have tried aluminum foil, plastic, card paper (to cover the top) and it still drops??? He is in a large low terrarium by the way. Also i haven't seen a poo or pee in his cage for like a week any ideas as to why?

Lets start with how are you heating the enclosure and how are you measuring temps and humidity... As for the bathroom thing they go when they need to just like people, its not a set time frame.

A humidifier in the room will do wonders. It will be easier to regulate getting main room up.

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Judging from the symptoms that you're explaining and the setup that you've told us about so far, I would have to go out on a limb and say you're probably using 20 long with a screen lid and heat bulb. The screen lid and heat bulb are your problem. Your best bet would be to get an under the cage heater and a thermostat. If you cannot afford to buy a handful of moss I don't know how you can justify a pet.

Short term: take a shallow, plastic container and put a clean damp synthetic sponge in it. Put it in the container in the tank. Wrap the top with foil to retain humidity.

Long term: ditch the fish tank and get a tub with a UTH and thermostat. Hefty makes an equivalent to the "40 breeder" for under $20, it's a 100 quart plastic tub with a locking handle in the center of each of the four sides to hold the lid down securely so your BP can't take any unauthorized slithers. WalMart and Lowes both carry it.

You certainly can make a glass tank, heat light and screen lid work perfectly fine, you just have to take steps to make the enclosure work... I have a giant 50 gal tank with a screen lid that i heat using a 100w infared bulb on a dimmer and measure everything with digital thermo/hydromometer.... Start by cover your entire screen top then on one end cut out a whole slightly bigger then the hood of the light. next on the opposite end cut out a rectangular opening, size and location of these holes will depend on your tank so youll have to experiment to get your locations correct, next youll need as big of a container for water as you can fit, i use 1gal ice cream buckets i cut down so the snake can easily get in and out along with a hide under the heat source, the water should evaporate over time and raise humidity in the enclosure.

But tubs, racks and pvc cages are all much better options. They all hold heat and humidity better, are much easier to clean and really a properly set up tank that holds heat and humidity is really wasted on a ball python who will just hide 90% of the time anyway.

+1 on covering over the screen top. Cover it completely with foil, really taped down around the sides (but no tape on the inside where the snake could theoretically touch it), with a hole for the lamp hood. You can then poke small ventilation holes in the foil as needed, but probably the big hole for the lamp will be plenty.
Another thing you can do is use a very large water dish (a dog dish or something that the snake can't tip) with a dark-colored bottom and set it right under the heat lamp.

One way to make longer-lasting humidification is to make something like the "dampit" things that players of stringed instruments use to regulate the humidity in their cases. You poke a bunch of holes in the lid of a tupperware container and put a damp sponge and some water in there, and put it in the cage. You could even hang it from the wall, and then the snake won't pee on your sponge. The water in the container will evaporate into the enclosure's air more slowly than if the container was sitting open, so it will keep the humidity more stable for a longer period of time and you won't have to refill it as often. In a larger tank you might need a larger container or a couple of smaller ones.

Do most of what is suggested here on my tanks, and then I add moss to the corners and around water bowls, thoroughly dampened to ensure it will help humidity and be more likely to stay in place.

Currently I have all my snakes in bins and have no humidity problems whatsoever. I spray them when I get slow but that's only once every 2 or 3 days.