Revision 2.massive. I kept the main structure the same but...
-Added BLTouch support
-Decreased lower belt holes for strength
-tweaked heatsink hole to allow better bowden clamp designs
-tweaked forward fastener screw holes for better alignment
-cleaned up dangling model parts
-added massive rigidity to the front to prevent wobble
-Increased size of recesses for easier sliding block installation
-fixed m3 nut holes. They are now snug enough not to fall out and loose enough not to require heat to get them in. Simply use a screw to pull them inside.
-Fixed fillet faces so the belts align properly
-removed mechanical endstop mount. Use sensorless homing. (Get a Duet already)

-this thing is now crazy rigid, and sits just a hair off the 2020 on the front, for maximum build area.

I recommend LJBrumfirld's Y mounts though I do have some qualms about their rigidity.

This is a work in progress. This has not been tested.

I was inspired to do this based on rothnic's non-linear rail project. here https://www.thingiverse.com/thing:2780420

I embarked down the road of linear rails for the y-carriage. The x carriage though, was bothering me. Only one side of support, no built-in belt alignment, limited hotend compatibility, and I would need to stack lots of parts to get it all to go together.

So I designed this. Did it in a couple hours but the spacing is about right for 12mm pulleys.

-28mm belt spacing
-Same width as stock plate
-Weight savings over stacked 3d printed parts with hardware
-Maintains stock x-stop.
-Compatible with front fan mounts for the CR-10 modular e3dv6 mount.
-Plan to add compatibility for side mounts
-Designed for MGN 12H carriage
-Keeps the hot end closer to the carriage compared to stock with printed parts. 1-2mm savings
-Uses common M3 hardware
-M3 nut recesses are designed to be tight so you are not pushing your nuts out.
-Low tolerance, only a couple tenths of a mil for slop so be sure to print slowly.

I plan to create y-carriages, corner mounts, and maybe motor towers to go with this. If you test it, please let me know the results!

Corner Bracket: https://www.thingiverse.com/thing:2788759

My parts are stuck in limbo so my printer is offline for the time being.

UPDATE: 2-9-18
Modified for MGN12H rails... which are actually what I ordered. Removed 4mm of extrusion on the mount after more carefully examining the E3d drawings. Yay! More clearance!

EDIT: 2-10-18. Tried to credit someone for design idea by posting a remix. Did not go well. I have removed my Y-brackets. Those will be a private project people can request from me directly as they need.

UPDATE 3-22-18: Revised a lot of things in V1.2
-Fixed hole spacing around heat sink. Left side was 1mm off.
-Added upper belt mounts. No more need to use the A frame and one of the screws as a belt mount. This also fixes belt alignment.
-Belt mounts with supports for extra rigidity
-Removed lower protrusion. It ws too low. When trimmed, it makes an excellent 2020 extrusion spacer but was overall not worth keeping with the added rigidity from upper belt mounts and braces.
-Increased resolution to ridiculous amounts. .001mm resolution on the STL with .01* angular resolution.
-Added a front bevel to support the popular Hurricane cooler for the CR-10 E3D mount. If you use the hurricane, be sure to use a very high static pressure fan.
--PRINT IN THE ORIENTATION PROVIDED WITH SUPPORTS. Yes, there will be a lot of supports to remove. No, the cosmetic scarring has no effect on the function. You need the layers to be perpendicular to belt travel for strength. Otherwise the layer adhesion is not enough for proper, ultra mega belt tension.