Screw a Bit of flat timber to the bottom of it, which is as long as the splitter, (use screws thru the splitter or stick some clamps on to hold it)
Now wrap it with cling film overnight to take the worst of the warps out of it! If you wish you can slide bits of timber Inside it to help shape it!
After 24 - 48 hours unwrap it again and use a hot air gun to finish reshapping it!
The timber and wrapping should take the worst of the warping out of it!
The hot air gun will help to Reshape the small imperfections left and will reset the shape!