Flattening Speed Ez-el

Does anyone have any tips for how to ensure that a Speed Ez-el is really flat? I bought one off ebay and it does appear to have a tiny amount of bow in it.

In college we had speed ezels in all kinds of wear so I assume there must be some relatively easy way to get them back into alignment/true?

I only use these for small prints, I screw them down to a nice, slightly oversized piece of 3/4" birch plywood, I put some little self adhesive rubber feet on the plywood to keep it from sliding. I use the Saunders single size easels more often.