Example of what can be done in today's web-browsers
Why an extra set of nuts?
While printing, the action of the pulleys rotating back and forth would unscrew the existing nuts and allow the strings to rub against the arms of the printer. Not in the mood to have lines snap or possibly come undone I drafted up a solution to add just a bit more friction to the threads.
Print in a set of three, drill the inner hole to just under the diameter of bolts, then tap with M3.5 threads, screw over existing nuts after pushing pulleys back against tensioning knob slider; leave enough play that the pulleys are not being squished.
Layer Height: 2 mm Infill: 20 % Top/Bottom Layers: 5 Perimeters: 4 Skirt Loops: 2