No BOM right now, here is why:There are at least three obstacles to overcome, which requires some machinery.Most worm gear sets with module 0.5 have drive worms with a 2 mm bore. It cannot be widened to the 4 mm of the stepper shaft as the overall diameter is too small. So you have to machine down the stepper shaft to 3 or 2 mm or manufacture a small adapter with a 4 mm bore and a 2 mm shaft. The usual drive gears (MK8 ..) feature a 5 mm bore. This would require  a 5 mm shaft, ball bearings with 5 mm ID and widening the worm gearwheel bore to 5mm ...  parts would become quite bulky and heavy. I took a 3mm shaft and made a bushing with 3 mm ID and 5 mm OD for the drive gear. The design offers not enough width for a drive gear with a set screw. I removed the half with the screw and glued the remaining part to the shaft.You need at least: NEMA8 stepper, a worm gear set 1:40 with module 0.5, a drive gear (MK8 etc), 2 ball bearings ID 3 mm for the gearbox, 1 ball bearing OD 10 mm for the pressure wheel, an idler arm spring, 2 pieces of a 3 mm shaft, 4 M2 screws for the stepper, a 4 mm bolt for the idler arm, and a hotend like Merlin, Prometheus, E3D ...Settings on an UMO:approx 3040 steps / e, eight step microstepping, retraction 0.4 mm @ 5 mm/s