With the addition of a Z-Axis the machine is starting to really come together. The main supporting column is a 4x4" square extrusion with 1/4" walls, mounted at 30 degrees. I'm hoping this will be stiff enough to be mounted cantilevered. It mostly passes the 'grab-and-pull' test, but I can feel it deflect a bit when I yank on it. Hopefully the forces during machining will be low enough for this to not be an issue.
I used two Newport 360-30 angle brackets to build the main support. These normally retail for about $110 USD, but I was able to pick them up used for $38 each. Here's a detailed shot of the lower mount.
This still makes them the most expensive part of the machine, but even with them, I think the mechanics could be built for about $100 per axis, including motors. I looked at a number of options for the supporting column, including welding a custom bracket, but all quotes came back at $300+. This cost less than $100 (including the column) and is pretty stiff. Plus I can re-use the brackets later.
I still need to tweak dimensions a bit. As can be seen in the photo above, the Z-Axis is a bit low and doesn't provide much clearance over the XY table. I will probably add a spacer on the angle mount and drill/tap a few new holes to get a bit more clearance. I'm only shooting for about three extra inches. This should give the final machine a few inches of working depth, including perhaps a small vise on the table and a cutter. I may also flip the Z-Axis around and mount the plate rather than the carriage to the supporting column, which will give a bit more clearance but has other tradeoffs.