What to do with a warped panel?
 

I am now completely done with the drop front desk I have asked about 20
questions about. It looks great except...

The drop front is about 30"x18" and fits flush in a frame when it is closed.
At least it is supposed to fit flush. The panel warped a bit and one corner
sticks out. It is no more than an eighth inch, but its not right. I
thought about planing it down to fit, but decided it was better the way it
was.

The customer didn't even notice the problem, but that is not the issue.

Should I ever be dumb enough again to try to get a large unsupported panel
to be in perfect register with a flush frame, what could I do differently?

Using plywood is the obvious solution, but is there any other way prevent
the problem? Maybe leave the frame a bit oversized, and plane it to fit the
the panel? Naw, that can't be right. Well, any suggestion would be
appreciated.