Mount plywood panels for painting

I started giving painting a real try. After painting in acrylics on cardboard I decided to go bigger, using 80x40cm 5mm thick plywood panels. The problem I encountered was that the panels were warped, I could not get any totally flat panels.
You can see on the bottom edge how the panel is bend.
So I decided to mount the panels on a frame. Having only a few tools on hand, I glued 40x28mm wood slats to the edge of the panel.

Laying the panel on a flat surface I connectes the four slats with metal plates.
The panel came out pretty flat. The added depth makes it look good when hung on a wall.