How can i fix a slight curve/ warp in black HDPE regrind boards?

I had a bunch of HDPE regrind 1/2 thick "pallets" made for a project. They are 24" x 33" x 1/2" thick. they really need to be almost perfect flat but some of them have a slight curve to them. I tried placing them on a hard surface with significant weight on stock of them for several days, but the.distortion is still there.

Some appear to be worse than others, but none of them have more than ~1/4" curve over the long side.

At a loss and looking for some advise on how to get them flat.

edit: To add, they were cut out of new 48"x 12' sheets.