WOULD LOVE TO SEE MORE MAKES, PLEASE POST A MAKE IF YOU DO GO THROUGH WITH THIS.
DISCLAIMER: my printer has been heavily modified and there may be things that won't work on a stock CR-10 or Tevo Tornado. Please plan ahead make sure you cover all your bases. This mod probably isn't for people new to 3d printing.
This is a set made to replace your X and Z axis with linear rails. I did this on a CR-10 but I'm sure it can be adapter to many 2020-based cartesian printers. Just get the appropriate length rails.
1 x 400mm rail with a mgn12h carriage
2 x 500mm rails with mgn12c carriage
-(or 2 x 500mm rails with mgn12H carriage.
2 x Delrin Anti-Backlash Silent Nut for 8mm T8x8 Lead Screw Trapezoidal ACME
Link to item on manufacturer webpage:
-(or use the adapter "Lead Screw POM Nut Adapter v1.1.stl" it's untested but should work).
Many M3 and M4 screws and nuts.
The carriage is modular but I've only made an adapter for a Titan Aero. It's should not be hard to attach just about anything to it since it's just 6 screws. More pictures of this setup (https://www.thingiverse.com/thing:3172348)
Lastly I want to reiterated that I can't guarantee everything will work on your CR-10 or Tevo Tornado. My printer has been heavily modified. One example off the top of my head, You'll have to figure out endstops on your own, I'm using a Duet Wifi for sensor-less homing.
There may be far more things that you'll have to figure out but I wanted to share all the work I've done.
The idler mount to idler clearance is pretty tight. I recommend using sand paper to smooth it out until it slides easily smoothly and doesn't get stuck.
[UPDATE] Created a purpose built idler/tensioner to go with this.
[UPDATE2] added "LeftX-1.31.stl" with a properly sized recess for the stepper collar to fit into.
[UPDATE3] Added a brace/guard around the front left on the X axis. Also added a couple renders.
[UPDATE4] Added an option to use MGN12H for the Z direction instead of MGN12C.
[UPDATE5] Added "Lead Screw bearing cap.stl" it was difficult to print in PETG as one piece so I split it in two pieces. This version prevents the lead-screw and bearing from rattling around while printing.
[UPDATE6] Added POM nut adapters, 18mm hole spacing and 16mm hole spacing. This should allow you to skip buying the one I listed.
[UPDATE7] Added x-idler v1.1.stl which fixes some tight clearance issues.