won't lie it is an interesting way to do it
Potentially you could make it more compact if you switch to gears, but I know on FDM 3d printers they are not the easiest thing to print you would almost need an SLA to do so with any degree of accuracy.
If it took a standard 540 can I would say HPI has a part that does exactly what you did, but I highly doubt it would fit in your rig.
You actually might want to switch to belts instead of rubber bands as after awhile the rubber bands will start to slip as they will stretch out where the belts would last longer.