Go to a welding shop and ask them for 2 plates of A36, 12" X 12 inches
and at least 1/2" thick. If they have anything the is close, but still
small enough to fit under your press, ask them if you can borrow them.
Take them home and try them out. Since your press may not have the
power to flatten the whole part at once, also try to get a 6" X 6" piece
of plate. Place your workpiece on the big plate, then put this smaller
plate on top, squish, move the work, and squish in the next spot. If
this works, you may have to bevel the edges of the smaller plate so it
doesn't dent the workpiece at the edges.

I have flattend 4" X 16" pieces of 16 ga steel successfully with this
method using my 20 ton HF press.
http://www.spaco.org/Press.htm

Pete Stanaitis
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Will wrote:
I purchased a cheap hydraulic press to help flatten out warped pieces of
soft metal. Apparently the press does not come with a plate or die set.

Can someone recommend a cheap attachment for a 12 ton hydraulic press that
would work on soft metal, and I need a pressing area about eight inch by 12
inch.