Board Making

I have been making boards for a while now and I am wondering about other plein air surfaces.  These boards are easy to make, inexpensive, sturdy, archival, and are suitable for pastel, oil, or acrylic.
I make the boards using acrylic paint, pumice gel, and water.  I do not have a formula for the ratios of paint to pumice gel to water.  I mix up my approach every time I make these boards.  I use a good 2 side 3ply plywood that is sanded and cut down from a 4'X 8' sheet down to various sizes.  I usually work on 2'X3' and 2'X2' boards.  I am going to start mixing up my board sizes so that I do not get stuck in a rut with my compositions.
I choose the color based on the lighting the day I am painting.  The amount of water and pumice gel is determined by how much tooth I think I am going to need for the painting.
I use a plastic paint tray and 3" paint brush used for house painting.  I put the water in the tray and add the acrylic paint and the pumice gel.  I mix to a consistency of pancake batter.  I prepare 3 to 4 boards at a time.  I usually apply 2 to 3 coats of my mixture to the boards.  The boards take about 10 to 15 minutes to dry depending on the temperature.