A Dune-style stillsuit is thermodynamically impossible. You can't both capture water and use that water to cool you via sweat evaporation. If you let it evaporate, it has to
leave; if you capture evaporated sweat you also recover all the heat that it took with it. Those suits are equivalent to going out into the desert with no ability to sweat, and rather than extending your life, would kill you much more quickly.
If they were externally powered you might get the numbers to balance, but they are explicitly presented in the book as powered by the human inside, which subtracts even more time from how long you're going to last in the desert before you die.
You can build a larger thing that recovers your water and cools you via some other method that uses external power, but I think you'd be hard pressed to ever beat just bringing more water with you. It won't be long before you're spec'ing a vehicle and not a suit... and then that vehicle should probably just bring more water, too.
On the more positive front, there is an interesting technology for potentially cooling the Fremen in the middle of the desert that could be based on something real: Paint that cools you by dumping your heat directly into space. Here's a video of it in action and what you might call a prototype of a "suit" that works like this: https://www.youtube.com/watch?v=FnKNOPlR2Yo While that YouTube video shows off someone using that paint on clothes, it seems pretty likely that that would not last very long. Paint on clothes is exactly as silly as it sounds for a long-term approach. But hypothesizing that someone could make clothing or suits based on this approach has the advantage of not being thermodynamically impossible, as evidenced by the fact that at least one substance with these properties actually exists. On Earth, that suit won't work in cloudy weather, but on Arrakis that's not a problem. Tapping the local human power to drive some circulation of either air or a bit of liquid cooling attached to some lightweight fins or some other sort of surface area on your back or something and you might just get a suit that could hugely extend your ability to loiter in a hot desert environment. You'd still need water, but much much less, or, the same amount could take you much farther.