Just by luck I found some PVC L molding laying around that turns out to be a perfect size. It's crusted in acrylic paint, but can be cleaned up. I cut off a piece to experiment with.
I was able to glue a matchstick using Superglue. - but it was a nightmare trying to get it in place, and the plastic has to be really clean.
But I discovered you can lay down a bead of hot glue quite easily and it sticks! If I go with the hot glue I will do a much neater job than I did here - this was just a "proof of concept" experiment.
An "L" that is an inch on each side - like this one - is the maximum size you can use in a 1 1/2" pipe - You could use a thinner metal molding and have more room for paint - and of course you could use a bigger diameter pipe. But I think this is a good size for me.
My palette is 18" wide, and as you can see I can put out all my colors without dividers and keep them separated. Also I would put white on the palette and not take up space with it on the tray.
So I'm first going to try an 18" tray without dividers. The hot glue strips don't look like they'd do much anyway.
So all I have to do is clean up the molding and cut it and the pipe to size and I'm there. The real test will be to see how it works out in the field.