In the past I have not been enthusiastic about painting on site with acrylics since they dry so quickly even in the studio, where you have much more control.
Why would you want to? (1) For work that is considered "finished" enough to put on the wall of galleries. Many galleries will not accept watercolours or any other works behind glass. This includes most galleries in Europe, as well as in the southern U.S. Oils are also considered more "finished" work but take much longer to get to gallery walls because they dry much more slowly. (2) For working away from your studio and yet wanting more finished work. For example, if you are painting in Spain for a month or more and want something that is immediately available to the collector, acrylics is your best bet since you can pack them around almost immediately.
I have painted on site from time to time with acrylics using what they call a "Stay-Wet" palette. Daler Rowney makes one that works well.The palettes are made from plastic and consist of a base tray with a tight-fitting lid. A wet piece of watercolor paper (or thin sponge) is placed in the base of the tray to serve as the water reservoir. On top of this is a sheet of grease-proof or baking parchment paper, to serve as a membrane to stop all the water going into the paint immediately. You lay your acrylic paints out on top of the grease-proof sheet. As the water in the acrylic paint evaporates, it is replaced by the water being held in the watercolor paper so the paint doesn't dry out as fast as normal. This is made for the more traditional fast-drying acrylics, but "the times they are a-changing"
The Golden Paint Company has come out with something called "Open Acrylics" that dry about 10 times as slow as the regular ones. I have used this with some success. They take some getting used to since thay are a bit more tranparent but the beauty of them, even in the studio, is that they are far more open to manipulation, allowing you to gradate areas without streaking, especially in skies or large areas of snow, etc. In the painting, "Dining in the Cathedral", (on page one of this website's original paintings and shown above) I did almost the entire painting with Open Acrylics. They mix with regular acrylics which lets you pick and choose between the two.
Give them a try. Golden sells small starter packs to let you do this. Honest, I don't own stock in the Golden Company!!!