Continuing experiments with different construction methods for the jacks. I calculate that these will need to occupy a total width, each, of about 3/8", including the clearance to the next jack in the column. The basswood I'm using is about 1/8" wide, so there's room for a bit of widening on each side, but *not* enough to use another segment of the same wood on each side to hold the axle for the tongue (i.e., as if the jack were made of 3/8" rectangular-section wood, with a slot cut in the middle, like conventional jacks). I could potentially work with some wood (which I don't have on-hand at present) that was 1/4" wide, cutting out the (slightly over) 1/8" slot in the middle for the tongue. In one of the experiments, I simulated this by whittling down two side pieces and gluing them on. Other alternatives are having only one side piece (I have yet to get this design to work very well, for assorted reasons), or using aluminum as I did in the original too-large one. This last alternative seems to be the best so far, in terms of ease-of-manufacture and in terms of reasonable stability of the plectrum. However, none of these really have extremely confidence-inspiring levels of stability. There's a lot of side-to-side play for the plectrum -- which I hope is a relatively less-critical direction, but any play has the potential to be problematic. Rotation about the long axis (of the tongue relative to the jack stem; rotation of the whole jack in its guide is another potential source of, effectively, the same problem) is reasonably minimal. And most critically, it seems that the front-to-back motion of the plectrum is well-defined and not compromised by too much play: the tongue swings back easily against the nylon "bristle" (return-spring), but when it comes forward it seats in a definitive position which seems quite stable and repeatable. And when pressed downward by the string, the plectrum and tongue "dig in" and hold all the more firmly in the forward position, which will give a strong and consistent pluck -- as opposed to the string force tending to make the plectrum retreat backwards and the tongue begin to swivel, which would be exactly the motion we don't want.
It's hard to see the details, a diagram would probably help. The aluminum moves with the tongue; a steel wire passes through for the axle; its ends are bent over and they cling in place to the sides of the aluminum piece, through friction (the axle moves with the tongue and turns in the stem piece, opposite from the larger jack where the axle was fixed in the stem). A tab coming off the back of one side of the aluminum, is bent around the back to form a back-stop, limiting how far the tongue can swivel (to prevent it touching other strings or jacks during its operation). And because there isn't really room "up inside" the bent aluminum for a spring, the way it was done in the larger jack, the spring is now located down below on the stem, angling out and upwards from the back face and hooking under the backstop tab (you can see the black duck tape that holds the ends of the spring, on the front face of this jack, below the aluminum). This has the benefit that the nylon spring thus cushions what would otherwise be the impact of the tab against the wood of the stem, helping to reduce noise. On the other hand, since the arcs of motion of the spring and the aluminum are opposite, there is a sliding motion of the tab against a small length of the nylon: which presumably causes friction though it's not a show-stopper, but more worryingly, may cause the nylon to abrade away. If I persist with this design, I'll probably have to take steps to ensure that a smooth portion of aluminum contacts the nylon, not a sharp edge. There may also be a way to extend the guide-wire, not present on this prototype yet (but visible on the large jack), so that the guide-wire and not the aluminum tab is what the spring pushes against.
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