First, thank you all for your suggestions and help.
Here is my milling corner as of today, with a stand for the X3 mill approximately as discussed - 2x2 legs and 4x2 top frames, all morticed, well glued and screwed, with 18 mm flooring board top and sides.
I was going to double up the top, but it doesn't seem necessary in this case as the ends of the mill base rest over the frame.
I think 2"x2" timber is a bit too small for the legs though, it does wobble a little, but not too much. I'll use 3"x3" when I do it again - I'll eventually use that stand for the BCA, and make a new 3"x3" based stand for the X3.
The BCA is in bits, as usual - the workpiece on the X3 table is going to be an adapter for CNC converting the BCA.
The beige thingy under the stand is a 4 lpm DeVilbiss oxygen generator, good for brazing and small-scale welding. It also provides almost silent compressed air, though only to about 80 psi, but enough to drive the alternative 60k rpm air spindle for the BCA. It will also eventually serve as a source of oxygen for my liquid oxygen making plant when that's completed.
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Thanks all once again,
-- Peter Fairbrother