Hi Pippin

X and y positioning does not need to be done to CNC milling machine accuracy standards. the pecks cut a small circle and can overlap each other enough to allow for any positioning errors.

Using a rail that will later be used on the machine (it needs to be a profile rail if only one is used round rails will work but two will be needed) the cutting head will be mounted on one of the rail runner bearings that will later be used on the machine. The cutter head will be small, not a gantry across the machine. At a guess maybe a 250mm cube should be enough to contain the components. made from laser cut and 3d printed parts.

If the rail used is not ordered a little extra long the cut length will be equal to the length of the rail minus the width of the runner block, not a huge problem the small section at each end could be hand finished. Or the rail could be ordered a little extra long to allow for this.

How to mount them?

My plan is to clamp a piece of say 90 x 45mm (4x2" imperial) dressed timber on the side of the frame member to be leveled this would need to be manually straightened to within a mm or so, not hard with simple tools Or better still buy a good straight piece from the local building materials supplier

I plan to mount the rail on this timber with stout wood screws!

This is where the laser or wire alignment system comes into play. The rail and cutter head assembly
only needs to provide solid support for the cutter head. Positioning the cut is done by the measurement system not the frame.

The X axis is possibly the weakest link in this design, as is as long as a simply flat area is all that is needed it should work fine. If however you wanted to pilot drill the mounting holes it would not be sufficiently accurate without setting up the laser reference or wire reference at 90 degrees.
Doable at the expense of complexity, probably unnecessary.

Regards
John

PS: Yes Pippin the epoxy mill begs me to finish it. 2019 was a big year for me work wise with a heavy smattering of other issues. I hope to get back on it soon.