Due to the design of the belt system, 2 of the belts connected to the extruder are not aligned perpendicular to the direction of travel. This causes a change in angle, which causes a change in belt length. I did a little simulation of this and due to the significant angle offset, the belts technically stretch over 3mm when moving from the center of the gantry to the edges. This is not good, and would put huge strain on the belts, pulleys and steppers, especially if you tensioned your belts when the extruder was in the middle of the gantry.
This is a little solution I cooked up that straightens the belts to be (almost) perpendicular to the direction of travel, while still wrapping round the original holes on the extruder plate for strength. It's not pretty, but its big and strong.
I would recommend printing this in at least PETG or ABS for the strength.
You will need;
1x "x5s belt alignment main r1"
2x "x5s belt alignment wheel r1"
2x M4x30 bolts (or 35, 40)
2x M4 nuts (preferably nyloc)
No supports required, just print as oriented already in the stl files.
To install it;
1) Assembly all the printed bits using the nuts and bolts, use pictures for reference.
2) Attach and clamp the 2 belts that DO line up properly to the extruder plate
3) Remove the 2 top wheels from the extruder plate
4) Put this assembly in place, push the top wheel bolts back through and assemble the plate onto the gantry again.
Note; You may need to bend the pins of the x-axis microswitch, and rotate the cooling fan 90 degrees so the wires do not get in the way - see pictures
Note: When tightening the 2 top wheels, you should push up from the underside of the extruder, so it grips tightly on the rail.
5) Feed the remaining 2 belts round the gaps in this assembly, loop them back through original holes in the extruder plate and all the way back round to the front, where you can zip tie them.
That was a terrible explanation, but just use pictures for reference :)
Hope others find this useful anyway.