Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4792341.
Dual Z without the hardware. An additional belt to support the far end of the X gantry. Particularly useful to reduce ringing in direct drive setups and improve consistency of bed leveling.
Requires ~700mm x 6mm timing belt and a couple of spare T-nuts that came with the printer.
I haven't gone to great lengths to ensure a perfectly straight belt path but with a few quick tweaks these belt holders should do the job.
Actual GT2 Idler pulleys may also improve the design, mounted on elongated V slot wheel bolts, for example but that will cost extra.
The belt path runs from the bottom left frame corner, straight up and around two pulleys on either end of the X gantry and straight up again to the top right corner. This path is constant length as the gantry moves up and down but provides additional support for the cantilevered arm.
I've been taking a few measurements to see if this mod has merit (or dual Z in general). There indeed is a measurable vertical deflection of the far end when moderate force is applied, easily 0.4mm up or down without too much effort - a range of 4 layers but only around 0.06mm deflection from the weight of a stepper motor alone.
The force of gravity or nozzle pressure could indeed cause some deflection, but this would be fairly consistent given the limited Z motion and would simply be compensated by bed level.
Where this does cause an observable effect is at the start of a print. A kind of hysteresis is observed, particularly if the far end rollers are too tight. The gantry moves down immediately before starting a print, then changes direction to move upwards but the far end lags behind. A calibration cube printed towards the end of the gantry exhibits a noticeable bulge akin to elephant's foot, but extending 2-3mm up the print.
This mod, or full dual Z can indeed reduce this effect by providing additional support to the gantry or pre-tensioning the far end (either up or down). Originally I had conceived of a symmetric two belt arrangement which could be tensioned, but I think a single belt setup could still provide benefit by balancing the springiness of the gantry against the belt tension. Effectively pre-loading the spring of the gantry and limiting further deflection.