These pieces were very simple to design. I had three main ideas I wanted to implement. The first was glue free bushing holders. My idea was to wrap the plastic around the bushings, so they get pressure fit within the cavity. You can still pop the smooth rod off, but now they align better and put more pressure on the bar.
The second idea I had was to flip the belt over. Doing so increases the z travel one could attain. The only issue is that the smallest possible nozzle from the top of the carriage will have to be 37 mm long. This is fine because it will still easily work with the makergear nozzles, which are generally considered the best in the United States. For the nozzle, I am doing something else though, which will be documented later on. It will be very close on architecture to the simple PTFE and brass nozzles done a couple of years ago.
My third improvement was bringing the bushing and nut closer together. This is part of the “optimization” I am making. bringing these two closer is a more efficient use of space and material.
Some other features I added were adjustable idler positioning, hybrid lead nuts and pressure fit x smooth rod. The adjustable idler positioning means you can get the belt perfectly flat, which is nice. My hybrid lead nut is as follows; a regular m8 nut takes most of the load, but for half of the bit underneath, it was printed as a 7mm hole which i tap to M8. The plastic puts enough pressure on the lead rod that we no longer need anti backlash systems. I think this is really going to be needed to get tested before I decide if it is better than a full anti backlash system like the Prusa Mendel. Last thing I decided to do was use pressure fit x rods. The Prusa Mendel uses an interesting system to ensure parallelization between the bars. What I am going to be doing is drilling these holes out to 5/16′ and pressure fitting the X smooth bars in. Again, this is something that needs to be tested.
I am calling these version 1. I already have a second potential motor mounting system which might be better. I might also use prusajr’s x rod clamp system if I decide pressure fit is not parallel enough. I already have a few ideas for version 2, so these are definitely not final yet.
S. J. Kelly 
Nice work. It looks like you and I have a lot of the same design goals. I was planning to try the X-ends with the bars at the top too, as I have noticed that a lot of space at the bottom is wasted.I'm not sure about tapping the plastic. It seems like it would wear out too quickly. You might be able to get away with a printed (but replaceable) nut in addition to printed bushings, but I'm not sure it's worth it. It would be better to have a short hexagonal tube, more like in prusa's design, but with a nut just sitting inside. This way, if the z axis lowers too far, the axis will lift off of its nuts the way the makerbot z-stage does, though using a tube rather than just a shallow indentation will prevent the axis from immediately being thrown out of alignment when this happens (the way the makerbot z-stage does).
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I think with the x ends at the top, it is the best. As for tapping the plastic it seems to be okay. Yes, it might be better to forgo the tapped plastic in combination with the lead nut. I really like how it tightened everything up a bit though. Originally I wanted to use a design similar to your Huxley leadnut retainer, but I could not easily figure out how to make it in combination with the X end and still be printable. Right now I am reworking the parts again and merging every file into one big config. I will probably release what I have now, so people can play, but next week I will probably get a version 0.2 together that works. Thanks for all the great designs and ideas. It helped a ton. 😀
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