This is an X-axis replacement I made for my Tevo Tarantula (but will work for any 2040 extruded aluminum setup).
I took the inspiration from http://www.thingiverse.com/thing:1580899 and modified it to be easier to print and use a single idler wheel instead of the 2 dinky ones that come with stock Tarantula.
BOM
1 - Idler wheel
1 - M5 25+mm w/nut for tension wheel
4 - M4 20mm w/nuts
* 1 of these should either be a flat headed screw - for the idler axle
* or you could use a thin wedge nut instead (just flip the idler wheel carriage before printing to make sure the screw head side is on the outside - if needed like on Tarantula)
Here's the [idler wheel](http://openbuildspartstore.com/smooth-idler-pulley-wheel-kit/) I use. *(these are readily available [cheaper](http://www.aliexpress.com/item/Delrin-smooth-Idler-Pulley-Wheel-wheel-kits-precise-CNC-for-v-slot-POM-Idler-Pulley-with/32654993164.html?spm=2114.01010208.3.183.iQ6Xkt&ws_ab_test=searchweb201556_10,searchweb201602_4_10037_10017_407_10033_406_10032_10040,searchweb201603_4&btsid=190a51de-52b9-4159-b0b8-d24e0172233c) as well)
You do not need any extra spacers for the idler wheel, this design has it built into the idler carriage.
Minimum supports are needed for the shell.