OK, here is a perfectly functional, very expandable (good for lots of panel sizes), quick and easy to construct, panel holder that uses binder clips to hold the top and bottom strips in a pressure position. Cheap? I used scraps I had on-hand. It works great, holds the panel in place while stabilizing it on the easel and allows for taping on photos if necessary. Reposition at will. Use the thickness of your panel for the top and bottom strips so you can paint over the edge.
The first panel I painted, I was surprised that previously I had trouble along the edges, but not now, and the darn thing moved all over all the time, not now. I might cut a hole at the top to make a handle to carry it.