The basic requirement of the mechanical design is to suspend the z-axis solidly 150 mm above the centre of the XY table with enough overhang to allow the table to move +-75 mm in each direction. Inspired by the shape of my bandsaw I decided to make a G shaped construction.
In order to get both sides precisely the same, I cut a sheet of 12 mm MDF diagonally and then nailed the two halves together. This allowed the original accurate edges to form the corner between the back and the base on both pieces.
I then drilled large holes in the inner corners and cut the shape out with the bandsaw and a jigsaw. I tidied up the cut edges with a bench sander.
I made the joints with 12 mm beading, PVA glue, screws and nails.
Here is the finished article.
And here it is with the axes fitted and a large set square for scale. I used this to check the orthogonality and it was spot on.
Unfortunately as soon as I had finished it I realised I had made a really basic mistake. All the right angles were braced by at least one sheet of MDF in its strong direction so I thought it would be pretty solid. However if I pushed hard against the side of the z-axis I could move it about 1 mm. Although this would be stiff enough for extruding plastic it would be no good for milling. What I had failed to take into account was that there is nothing to brace it against twisting around the vertical axis. Adding sides and a top to complete the outer box would have helped. Another idea I had was to fill it with concrete. When I thought about it a bit longer I realised this is why CNC mills are usually based on a gantry design. Reluctantly, I decided to scrap an entire weekend's work and start again without ever powering it up.