The 200mm x 500mm plate I ordered from Poland £66 worth) looks to be in a reasonable state. That EN2017 seems to be fairly machinable and is a nice flat, parallel thing. The markings came off easily enough with some IPA but here are the markings for the record.
Then marked it up and chopped it into 3 pieces by hand on the bandsaw, as they are too big to fit under the arm.
Having skimmed over the ends (200mm lengths), I had to spin them 90 degrees so I could ensure the faces are square. The larger piece for the baseplate is 270mm long, so not much chance of fitting that in the normal vise opening. However, the jaws can be repositioned to give an extra capacity like this. It's OK for light work such as this but I wouldn't use it for heavy cuts. All the clamping forces are taken by the two jaw fixing screws under tension.
This is the vertical plate, which is only 168mm high and 200mm wide. I can get the long (200mm) dimension in the vise by removing one of the jaws and selecting the appropriate ground parallel to keep the work almost horizontal. Used this arrangement for skimming the ends of the baseplate too:
By the time I'd machined back to a flat surface, the final dimensions ended up a little undersized. The nominal width was almost bang on 200mm but not entirely parallel. I will adjust the CAD model to accommodate the actual dimensions which are 166mm x 199mm for the vertical plate and 199mm x 270mm. At least the 199mm dimensions match. That means the width will come down from 200mm to 199mm and the height from 168mm to 166mm. That's not the end of the world, as I pulled most of the dimensions out of my ass.
There. All squared up and deburred. Finished up to the nearest mm.
Right, once the CAD has been frigged accordingly, I can get on with machining these boys. I think I'll tackle the vertical plate first, as it takes the harmonic drive unit and the servo motor, so will give me a feeling that something is starting to come together.