Cant keep humidity up in my 40gal breeder tank

I have a columbian boa and i am having issues keeping the humidity up and need suggestions.
i use cypress mulch as substate, and when i mist, it goes back down to 45% humidity within 45 mins.
i use a 100w ceramic heater attatched to a thermostat that keeps the basking spot at about 92 degrees wth an ambient temp of about 80 degrees.
i tried a larger water bowl and moved it in various spots around the tank with no success.
i have damp sphagnum moss all around the tank.
i covered 75% of the screen top with plexiglass.
i have a humidifier in the room (and i already live in florida so the humidity is high)
should i try to use a UTH?
i heard UTH’s dont eat humidity like a ceramic heater, but i also heard it doesnt warm the air to make a temp gradient? Is this true? Should i try to use a UTH as a primary heat source with the ceramic heater on the thermostat just to aid the heat and make a gradient? If so how should i do it?

I prefer to keep my snakes (mostly colubrids these days, but I've had a BCI & at least 8 BPs in the past, plus+) in glass tanks w/ screen tops. To keep the
humidity up, and the heat actually in the tank, you'll need to work a little harder...that's all. I'm also using UTH on all tanks, with overhead heat as needed.
Heat rises...so yes, UTH DOES raise the ambient temp. in a tank, but if the glass isn't insulated & there's too much breathing room on the top, it's all going
"up the chimney" so to speak. Many snakes do seem to appreciate the "belly heat" for digestion, but there's more than one way to achieve what a snake needs.

You have to realize that when others are talking about their plastic enclosures, those enclosures have MINIMAL air exchange...that's what's keeping the humidity
inside. I've tried a couple of Pro-lines (plastic cages) in the past, they're really not to my liking (& I sold them). So to make your tank work the same way, you'll
need to block much more of the screen top...at least 95% or more. I'm not kidding...not all plastic cages even have vents in them...some only allow air exchange
via the tiny gaps around the doorway.

Think in terms of efficiency- heat rises. So while UTH may need to run at a milder temperature (so a snake isn't burned by laying over it) all the heat is rising INTO
the cage, whereas with overhead heat, that heat is rising too, so only some of it's going into a cage. Glass or plastic, neither one is all that "insulating", but it's easy
to make a glass tank hold in heat better. And don't forget the bottom...if you use UTH, it has to "breathe" a little (for safe operation) but otherwise, insulate under
the tank, and the back & sides of the tank. You can even insulate the lower part of the front. Snakes like privacy anyway...if you want the cage to look better, first
install some scenery around the back & sides, so you won't even SEE the insulation. Depending on how cold your house or room is, many materials can be installed
to insulate glass, including corrugated cardboard, poster board (the kind w/ foam center), cork (either on a roll, about 1/4" thick, or tiles), foam board, carpet tiles etc.

If you insulate the tank & restrict the airflow, both humidity & heat will stay in, & snake will have privacy as a bonus. Simple enough?