Ally plate boat warpage

[NTGeorge]

For the brains trust, I'm welding a plate ally box type mounting for a Minn Kota onto the gunnel of the current boat that I'm making. however when the top plate was welded onto the gunnel it warped the top gunnel sheet quite badly. I was prepared for some warpage and movement but this was much worse than I had expected. Is there a way to fix this? I know with steel I would just run a weld underneith the warped pannel to straighten it some what, but I havn't been making boats long and am a little unfamiliar with straightening processes with ally.
Any help on this subjust would be muchly appreciated

Cheers