I usually paint in oils but do use acrylics and have worked on learning more and more with them outside. As you know, keeping them fluid is not always easy in the summer. I tried the stay wet pallet - great inside but not great outside in summer. One thing I tried that works pretty well and is FREE is corrugated cardboard. The regular brown box stuff that's everywhere. I cut a piece and place it in a shallow plastic tray and spray it wet and add some water to the tray. Then put my paint piles on it. The cardboard doesn't fall apart and the openings allow water to collect and keep it moist. Naturally it gets thrown away later. It seems to hold up pretty well for a 3+ hr session. If I need to walk away for a breather, I spray the pallet and use a grocery plastic bag that I can lay over the pallet (also FREE). OK some paint sticks to it but all things considered, it works well. You are using a brown colored pallet however.
I try to calculate my piles of paint so I use most all of it since it will get discarded. I think that's just part of the deal when using acrylic outside. BTW: Golden Open paints will stay moist for over a day. The catch 22 here is that sometimes I want backgrounds to dry. You can mix the Golden Open with regular acrylics too. I tried the retarders but they don't seem to do that much. I figure if you can learn to deal with wet oil paint on your work, you can deal with drying acrylic.
That's a great idea using the corrugated cardboard. I like the Golden Opens too and also the standard for stuff I want to dry fast.