I've been using a glass palette when I paint outside and my acrylics often dry very quickly in the sun. Here's a tip thanks to Jay Babina who sent me an email. She writes:
I was painting acrylic outside and was going through the usual spraying the pallet routine etc yet once the sun popped out, my mixtures were literally drying in minutes.
I investigated the stay wet pallets and tupperware pans etc too.
What I tried that worked out great is corrugated cardboard. I use a regular piece of glass as you do and just placed a wet piece of regular light brown corrugated cardboard on top of it. I wet both sides pretty good letting water run into the corrugations. It works great. It holds the moisture, doesn't designate or fall apart and best of all, it's free. A piece will last for hours or more with an occasional spray if needed - but nothing like glass where thin mixtures can dry in less than a minute on a sunny day. It actually traps water on the underside if on glass. When you're done, just throw it away. I know your pallet is fairly vertical in which case you may have to experiment with some tape etc.
Give it a try and see what you think.