To make the x-carriage I started with a 107 mm x 72 mm block that was 1.5 inches thick. After getting most of the shape I glued a 1/4 inch piece of plywood to the bottom to provide the mounts for the extruder.
On the left (front) side I drilled two holes with an 8.4 mm bit. The first one, with location in (y, z) of (12, 19) I drilled all the way through to the back right side. The second hole, at (57, 19) has to be drilled at least 70 mm deep, but not all the way through. From the back right side, I drilled a third hole that meets the second hole, but offset by a small amount. This gives some play to the distance between the rails and allows the carriage to move freely. This third hole is at (51, 19) and is also 70 mm deep. The intersection of these will be revealed when the block is cut down.
Also on both the left and back right sides I created slots to allow the belt to get inside the block to where the clamps will be. These are square holes at (y, z) (24.5, 27) – (27, 34). They are 15 mm deep. I used the side cutting bit to cut them.
On the right (front) side, I drilled 4 holes for the x-belt-clamps. The location for these in (x, z) are (23, 18), (84, 18), (23, 31) and (84, 31). Each was drilled with a 5.4 mm bit to a depth of at least 35 mm. On the left back side I drilled four holes in the same locations, but with a 10 mm bit to a depth of 40 mm. These holes provide clearance for the screws. It is important that these holes are not drilled too deep.
There are 4 holes drilled through the top. These provide clearance for some of the carriage mounts. The loc
ation of these in (x, y) are (43.5, 45.2), (63.5, 45.2), (35, 39.5) and (72, 39.5). I used a 10 mm bit and drilled all the way through. I drilled two more holes, but they turned out to be unimportant.
Unfortunately, I didn’t take any pictures of the next few steps.
Now, I cut this into the shape that is needed using a hacksaw and the side cutting bit where the hacksaw couldn’t be used. I drew lines on the top at (y = 27), (y = 39.5), (x = 35), (x = 72), (x = 13) and (x = 94). There is a line going to each of the four holes on top. I cut through each of these until I reached the hole. I also made a short cut between each pair of holes so that the corner block came off. On the other side, I cut out u shaped pieces along the lines. The end result is a bit like a pitch fork with one handle in the center and then three prongs on the other side.
The middle prong has to be cut back. You can see the hole that goes through it. The hole should be completely cut off with a little extra so that the rod won’t touch it. This prong just provides a little extra strength and a glueing surface so it is not too important.
On the handle side, a slot needs to be cut for the belt clearance. This goes from the existing line (y = 39.5) to (y = 45) and is 19 mm deep. I went too far on this and the part broke. I fixed this by glueing it, but it broke again. Finally, I decided to glue a piece of plywood to the top to strengthen it.
The double hole on the handle side needs to be smoothed out so that it makes an elongated hole. I also smoothed out the edges of the vertical clearance holes, but this is just an aesthetic thing.
Next I made the mounting board which will be glued to the bottom. I used the existing piece as a template. Look at the image to see the approximate shape.
In the board I drilled holes in (x, y) at (23, 5.5), (35, 5.5), (47, 5.5), (60, 5.5), (71.5, 5.5), (84, 5.5), (27.5, 45.2), (35.5, 45.2), (63.5, 45.2), (79.5, 45.2), (35, 17.5), (35, 39.5), (35, 52.9), (71.5, 17.5), (71.5, 39.5) and (71.5, 52.9). These are each 5.4 mm and drilled all the way through.
I glued this board to the bottom of the pitch fork piece (the belt slots are near the top). After that, I mocked up the x-axis and made sure the piece could slide easily. There was some interference from the middle prong, which I trimmed back. 
The final part is the x-belt-clamps. I made these out of 30 x 14 mm plywood that was 1/4 inch thick. There are two holes in the middle of this that are 13 mm apart. That means the are 7 mm from the edge (center) and 13.5 mm from the other edges. First, I drilled a 5.4 mm hole all the way through and then I drilled a 10 mm hole half way through. I’m a little tempted to remake the y-belt-clamps in the same way.
