The X Axis:
I decided to go with ball bearing drawer runners for the X axis platform. They are compact, smooth, and virtually no play in them. I mounted them between 2 lengths of 25x25x1mm aluminium angle...
I did have to bend the end-stop tabs down so they could clear the screw heads used to attach the inner track to the inside rail.
This is where keeping the printer screws is a very good idea. I was able to use the self tapping pan-head screws which were a perfect fit between the "U" shape...no chance of moving up or down.
Aligning them was dead easy...use a set square on the front edge of the printer base plate and simply line the rails up and screw them down. I used flat head 8G x 12mm self tapping screws (pre drilling the pilot holes) I put 3 down each side, but the centre screws seemed to pull the rails down and bound up the runners too much. One at each end is adequate to hold everything in place.
There are a couple of places in the printer base plate that are pressed with ribs (obviously for strengthening) but you can put the rails close enough to the left/right edges to get full length support.
I drilled and tapped 4 x M4 machine screws along each side into a sheet of 6mm acrylic for the milling platform. There wasn't quite enough rigidity in the acrylic to stop the edges rocking when pressing down on the platform, so I also screwed 2 lengths of 25mm square section aluminium across each end...which firmed it up just nicely.
I needed to cut out a small section at the end of the back square channel to clear a micoroswitch that I mounted for the axis + endstop. This was a lesson learned when I crashed the channel into the switch and nearly tore the thing apart...bit of heart stopper!
Next....the Y axis.