I like foam core best and I use 24 x 36" sheets. But I've used poster board also and I just ignore the wrinkling.

For me, the point is to get several really large samples. I tape them up in different places to see how the color looks throughout the day. Those are lightweight. You can do a corner, tape it to the ceiling, do one near a window and one on the opposite wall etc.

I only do one color at a time as doing them side by side can distort, especially if they are muted.

TruHue likely has some type of coating so the paint sits on the surface and liquid from the paint isn't absorbed. But I don't find 12 z 12 large enough.