I would remove the top. On a nice spring day I would lay it on the
grass with the humped side up. Let it sit there a day preferably with
some sunshine. The piece has probably gotten wet at some point and the
top dried quicker and pulled the warp. I am suggesting you try to
equalize the moisture throughout the piece. I'm thinking you will see
some substantial improvement after a day of absorbing ground moisture on
the shrunken side and air/sun drying on the top side. I would them give
it lots of weight on a flat table surface, not on the concrete patio or
garage floor. After a week, see what you have.
[Dan G]

I like your suggestions. The lawn and sunshine are readily available. But the
only large flat surface is the garage floor. And weight? Hmm... 2 front
wheels of the Toyota come to mind...

Thanks.