How To Build a Cajon - Snares
I don't recommend the snare setup depicted here. Read on for more info.
Ah, the snares. Perhaps the strings weren't thick enough (classical guitar "A" strings), perhaps they shouldn't have been so far up in the corners (longer strings = more mass), perhaps my notion of the "snare" sound is different from typical cajonares, perhaps my tapa is warped; but the snares seemed ineffectual, even after considerable tweaking. Here, anyway, is how they are set up. The image on the left shows the tapa before screwing and the various pilot holes in the tapa and the internal frame. The red lines show where the tapa screws will hold it (there are only three along the top of the tapa, the middle one was already screwed in once, thus the hole looks larger). Also visible are six large holes in the frame in two sets of three (which will hold the zither pins from the inside) and, above each of them (actually below them in the image; one is indicated with the blue arrow) is a small hole through which will pass a snare string (i broke and lost drill bit in the string hole rightmost in the picture, thus a secondary hole had to be drilled). Leading from the exit of these snare string holes is a groove where the string will lie (as demonstrated by the yellow line.) The string passes along the inside face of the tapa, through the groove, into the string hole, and finally wraps around the zither pin on the inside-face of the hickory frame. On the other side of each string, it passes through a similar groove and through another small hole; it is simply knotted on the other side of the hole to hold the string. The grooves are cut to both prevent the string from pressing or bulging the tapa (which could also affect the seal) and so that the string is not unduly stressed as it is tensioned or released. It may be a good idea to grade the entrances to the string hole so there isn't such a sharp edge for it to pass over. I have broken one string so far in my snare tweaking, and it was at this edge. Werner wrote in to say that he used a temporary small helper board screwed into the frame to act as a "pretend tapa". He then used blocks to hold the strings against this temporary board. Once those blocks were permanently installed, with the strings passing over them, he removed the temporary board and installed the tapa, which in turn had a nice close fit with the strings.
There are many, many variables in the instrument that affect whether or not the snare works well, not to mention a wide variety of opinions as to what a good snare sounds like, so it's hard to say what kind is best.
Standard zither pins (a.k.a. "tuning pins") are available online or, better, from your local friendly music store. The proper way to use them is apparently to drill the hole and then hammer them most of the way in (screwing them in can apparently cause them not to hold their tune as well over time.) It's probably not crucial in this application since the snares will never be at particularly high tensions. Ocaña describes the tension as just enough to hold them against the face to cause a rattle. [read on, below...]
The rattle setup was a good start: an edgy, barky snare, but the rattles are, well, rattly, and the snare sound was still on the short side and lacking in higher-frequency shimmer. I took old "E" steel guitar strings (.52 or .54 inch diameter, not sure which it was), carefully unwound the wrapping from the core, and gradually made 10 5-inch long pieces that i joined with some hammered copper 12-gauge wire and electrical solder to form a somewhat fancier custom snare that went in the other corner (the strings were slackened and just pulled out of the way). Two zither pins were used to mount the snare assembly: the top pin could be used with a string tied to the strategic bend in the wire (near where the actual snares connect) as a tensioner of the snare, but it didn't seem necessary, and is not done in this picture. (The old classical guitar string snares with some rattles are still hanging around loose in the picture, they are not contributing to the sound.) A whole lot of time was spent straightening and tweaking with the very finest degree of accuracy the lie of these snares so that they sounded but did not rattle excessively, etc. The combination of the two snare systems sounded nice. (Note that the snare tweaking was done after construction was finished, since you need to test it with the real instrument, but it's included here out of order.)
However, the best snare system, in my humble opinion, is to purchase a snare drum snare, clip off one end of it (in so doing, you probably want to shorten it to an appropriate length, as they are quite long), and then figure a way to mount it in your cajon. This is what i have done, and it's my favorite system by far. Perhaps it's not a traditional cajon sound, but it's what i was looking for. I have mounted it on the same copper wire which i use to tension, position, and otherwise adjust the lay of the snares against the tapa. This alignment is crucial, and tricky, and requires tweaking, (and some re-tweaking, especially while traveling with the cajon). Perhaps there is a nifty way to have it lay correctly and never fall out of alignment. I seek this method, but have not found it yet. In the mean time, it's like tuning your guitar: a nice moment to get in touch with your instrument before playing. :-) Pictures of this system are forthcoming; it's essentially the same as my "home-made" snare described above, but a lot more effective. If there is too much tension towards the tapa, or too steep an angle, the snare wires curve up to the tapa and then continue bending and "reflect" off the tapa face. If the tension against the tapa is too weak, or angle is too slight, the wires won't make good contact. Remember to adjust this while the tapa is leaning back (if you play with a backwards lean).
There are, of course, many other snare designs for the cajon, (see the other links section, as well as the readers respond section.) Some use snares all the way across the center of the face, which i would expect to reduce the separation of the kick and snare sounds. Many cajones (such as those from Meinl) allow the snare to be disengaged completely, to allow a more hollow, ringing high sound. I didn't bother, given my personal intentions for the instrument.
In the further notes (english translation, without images), Ocaña says: "I hope ... that percussionists would restring their cajones, just as guitarists change strings on their guitars, to get their own, personal sound."