Hey John (if you're still reading..!)

With regards to the platen - would you size it so that it's full width passed through the rollers, or so that it was slightly wider that the rollers as in the image below..?

Click image for larger version. 

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Would there be any advantage of milling a couple of shallow grooves on the underside of the platen to enable it to be guided by the support bearings, or does it tend to ride perpendicular to the rollers with the plate/art/blanket in position..?

Click image for larger version. 

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Wal.