This is a reverse of the Sigma3D - Y axis Tensioner ( http://www.thingiverse.com/thing:2014251 ) to allow for it to be placed on the backside of 2020 extrusion.
This is modified for use with the TronXY X3 bed upgrade here - http://www.thingiverse.com/thing:2185364
Requires 4xM3 nuts and 4xM3 25mm screws.
The file with "center_nut" on the end should allow for the use of the original hardware.
UPDATE: v2 should counteract the pull effect on the top half by the belt.