Well the best thing I can tell you is to do a foil treatment on the top of the tank. The easiest way to do this is with a roll of foil tape. Cover 3/4th of the screen top (leave a space for the lamp!) with the foil tape.

There really is no reason to get a mister, those are more used for amphibians and very tropical animals. 50% humidity is possible with a tank, it just takes a little more creativity. What is your substrate? Cypress mulch is usually the best substrate for helping hold humidity. Make sure the substrate is not wet though. It should be completely try aside from light misting.

You can also get a bigger water bowl but do not place it over the UTH. This will only cause the water to evaporate quickly and will make it warm and a breeding ground for bacteria. Place the water bowl in the middle of the tank.

What kind of hides are you using? Some hides tend to absorb moisture from the air. I like to use plastic hides to prevent this.

I know your GF likes the idea of being able to see into the tank, but the reality is that ball pythons hide 90% of the time. They are rarely seen outside of their hides if they are happy and comfortable. I use iris tubs and they are actually very clear. I can see my snakes perfectly fine through the plastic.

If you are still struggling with humidity after the above recommendations, it might be a good idea to think about the tub. When it comes down to it, it is about the snakes well being, not your preferences.