I think at this point, most people use their hinged extruders without springs. Improved consistency in the filament diameter from reputable vendors has made this possible. Still, I wonder if there is anything to be gained from having springs. The only choice around here is a variety pack at the hardware store, and only 4 out of 50 springs are even remotely suitable for use on the hinge, so here is one alternative.

Since we want this to be a spring, we need to leav out any kind of fill, in favor of loops, so
set the bottom layers to zero
loops to some high value like 5
and Zero fill, but don't disable fill (skeinforge)

One side of the spring has a lip. This side faces out, so as to catch the head of the screws.

Even if this thing doesnt do the trick, it makes for another good tutorial piece. It demonstrates how thin features near the base can be printed reliably by adding the anchor disks. Also note the breakaway supports in the screw slots. Those are all meant to be trimmed off, to leave the slots fully open for easy testing and removal.