First I would talk to the boss and see if he has any input on the problem.
The reason it warped is because the machining operations relieved the stresses built up from the manufacturing processes. I have had some success in clamping only from the side or center and allowing the material to ''float'', but on a piece that size it would be difficult to do at best. Setting it up in 2 or 3 vices rather than clamping it down to the table might be an option.
You proposed the best answer: Sent it out to be stress relieved.