I fear you are going to be terribly disappointed with the results. If the metal you are flattening is narrow enough you would be far better served with a rolling mill. Not real cheap, but much more effective.
If you persist with the press, you'll just have to fabricate two heavy plates to fit.
...but don't expect the metal to stay flattened once you remove the 12 tons.
(Don't take my word for it. Give it a go. Then Google "rolling mill".)
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 us> I purchased a cheap hydraulic press to help flatten out warped pieces of
Going to your web site, I have basically the same Chinese press you do. Doesn't anyone make some kind of die set that would fit the ram of such a press as an off the shelf piece of equipment? It seems like a real hassle to to have somehow balance two thick pieces of steel, then get a third one between it.
My application is flattening rackmount rails on equipment or fixtures that slide into rackmount equipment. I need to have both hands on the piece I am working with and really wanted to avoid having to balance the plates on either side of the equipment.
Since most people don't use these for flattening things, and those who do are usually capable of making custom tooling for their needs, the answer to your question is "No".
Once you find whether you *can* do the job with your press as described, now is the time to build your own fixtures for attaching the plates to the press.
So -- make up a way to attach the lower plate to the support, and the upper plate to the ram of the press. There should be dozens of ways to do it, depending on what tooling you have.
And I'm not willing to make it for that -- especially since I don't have the press to test it, or the tooling to weld up the fixtures which would be the best way to do it. I suspect that the same applies to most of us here -- at least the "not willing" part. This is a hobby newsgroup -- we are in it for fun, not to make things for money. You might find someone who will.
Where are you getting rack-mount rails that are this badly distorted? The worst that I have seen could be fixed with a pair of duckbill vise grips. :-)
A piece of pipe that will fit over the ram with a set bolt to hold it on, welded on to a plate the size you need, braced to the corners and edges with triangular plate. Weld the bracing plates to the pressing plate and the pipe. Another flat plate for the bottom should do it. Hope that at least gives you some ideas.
There is no cheap attacchment to do this. A piece of 3/4" tool steel to make this part is $308 from McMaster. If you load a 3/4" thick 8" x 12" plate in the middle, with 12 tons, it will bend. If you are pushing on a rubber sheet, so the load is uniform, it will bend about .040" in the middle. If you have the lower plate supported at the ends, and push with 12 tons in the middle, it will bend about .100"
In order to help you , what we need are some pictures of whats bent, the metal thickness and where you want it trued up. A picture is worth a 1000 words. Ive been bending and straitening metal for 40 yrs and there must be a simpler way than using a press. Await the pics. then ill think up a simple way for you to do your job. Ted Dorset UK Master metal worker and problem solver.