Quick status update.

I've ordered a 20x12 vac table - that size suits the star mill fairly well. I was preferring the pressure foot solution, simply at the moment I use ground dowels to locate the board for 2-sided alignment, and don't fancy drilling the holes for the dowels in a vac table... but I'll figure something out. Discarded: DIY 3d-printed pressure foot - altogether too clumsy, and the £40 Chinese pressure foot (try finding one in the UK... everything is 3-4 week delivery - i.e. from China) - plus the thought of attaching the whole assembly without knocking the tip off the engraving bit scares me. That remains an option if the vac table doesn't work well enough (though I think it will).I reckon I'll need to add two edges to the vac table to align the board adequately. So, thanks to m_c for the recommendation (I've ordered from the links you kindly provided). Thanks also to the other suggestions - all valid in their own right.

Now... anyone know any chemistry for bright copper plating an activated PCB substrate? (I'm about to throw myself at the mercy of an electroplating supplier but there's too much secret-blend-of-11-herbs-and-spices to lock you into their supply chain).