I run into this problem myself from time to time, and it's hard to solve without having the parts in hand and without being an expert (which I'm not). When I get really stuck, sometimes just staring at the pieces, or walking away and then taking a second look will help.

The post you mention is probably the best place to start. Most likely, the spring is supposed to be installed with the two ends slightly closer together under tension, and that tension would slightly constrict the coil in the middle of the spring. Start there, and find posts that the spring coil will fit, then look for places to catch or anchor each end of the spring. Also, many times one end of the spring will be anchored against the inside of the case and there may or may not be a groove there. Also be sure to flip the spring over vertically and horizontally when looking for places it could go.

To me, it looks kind of like the spring that puts tension on an M/X flash sync switch and provides the "detent" for the two settings.