You want his warm side (on substrate) not over 90*, so tweak that down a bit for safety to 88-89*.
Those inexpensive stick-on humidity gauges are very inaccurate, & yes, the heat + humidity makes them come loose & stick to the nearest snake going by.
Misting doesn't do much for raising humidity...you might try making him a "humid hide". Get a plastic food container, cut a door-way in the top, & fill it with moist moss,
leaving enough room for your snake inside too. My snakes love to use them...they can smell the humidity & are drawn to it. You'll need to refresh the moisture at least
once a week, maybe twice. This will really help him for shedding too.
As far as covering the screen, you need to leave a little space for air to flow, but by laying something on top covering most of it, it doesn't have to fasten to the screen or
cage, nor does it need to be perfectly fancy...just so it does the job for now 'til you figure out a more attractive option. (you could cover a piece of cardboard with some
plastic coated "Contac" paper to keep the cardboard from absorbing moisture...anything like that will work fine)