Here's some pictures of one I built for for a fellow CPF member a while back.
I use
these 10A switches, a MagC tailspring, and a disc cut out of
PCB clad with copper on both sides.
Just drill a 3/8" hole in the tailcap to mount the switch. I use a 1.25" washer to trace out the disc on the PCB, cut it out a little oversize (the washer is a hair too small for a press fit), and sand down the edges to an exact press fit. I sand of the copper around the outer edge of one side, where the spring will go, so it doesn't accidentally make contact with the tailcap. And drill a small hole for the wire to feed through, placement isn't critical, just somewhere near the middle. Personally, I find it easier to solder on the spring first, even though it makes the later soldering of the wire a little more difficult. I sand the bottom of the spring flat with a benchtop 6" disc sander. But you can get the same result by laying sandpaper flat on the table and moving the spring across it. And you definitely want to pretin the bottom of the spring, getting as full coverage as possible. However, even if there are gaps in the tinning, don't worry too much. You can still get good results. I probably only had about 70% coverage of the spring used in the photos.
Now that the prework is done, assembly is relatively simple. Getting the initial bonding of the spring to the board can be a little tricky. Center the spring on the board and put your iron where it is contacting the spring and the board at the same time. You want it in a spot where you have good tinning of the spring. Use solder to make a two "heat bridges". One between the iron and the spring, the other between the iron and the board. The spring one will be a little more difficult since you are trying to heat more metal, so do it first. The bridge to the board should be fairly easy to form. Once both brigdges are formed, add more solder to merge them into one large bridge from spring to board. By large bridge, I don't mean a large blob, just a continuous bead of solder. And just a "spot bead" of solder for now, don't try todo the whole spring yet. Pull the iron and let that solder bead cool to firmly fix your spring in position. Then go to the opposite side and repeat the process. Except this time, once you form the initial bead, you can start moving the iron around the spring forming a long continuous bead of solder along the whole bottom of the spring. When you get around to your original spot bead, it will just flow into the continuous bead and you can keep going until you are all the way around.
This next step is optional, but I like to do it to improve contact with the tailcap. On the bottom side of the PCB (opposite the side withthe spring), I add a small bead of solder all the way around the rim of the PCB. This is done just like on the spring but is much easier since you only have to heat up the thin copper cladding on the PCB.
The switch soldering is very simple. If you want, you can shorten the wires as needed. But there is plenty of room to coil up any excess in the tailcap if you want to leave them long. Put one wire through the hole you drilled earlier and solder it on the spring side of the PCB. Depending on your iron size, you may find it easier to reach the wire end in the center of the spring either through the side gaps in the coil or through the through the larger hole in the center. I've used both techniques. Then just solder the other wire on the bottom, non-spring side, of the PCB. You now have a the guts of the tailcap switch ready to go.
To install the guts in the tailcap, just poke the switch post through the hole and screw the waterproof cover on to hold it in place. Then just press the PCB with the spring on it into the old spring groove in the tailcap. If the wires were left long, use a slight twisting motion as you install it to coil the extra wires around the switch body. Depending on how tightly you fit the PCB to the tailcap groove, youmay be able to bottom the PCB in the groove just by pushing. But if it's a tight fit, I use a piece of PVC pipe to fit around the spring and drive the PCB home.
Edit: The same thing can be done in a C sized tailcap too. You just have to sand down the corners of the switch very slightly to make it fit. Don't worry, there is plenty of plastic there to remove what you need to. And the switch hole has to be off center since the post is off center of the switch body. The springs I used were identical in style to the C springs, but only about 3/4 as big. I got them at my local Ace Hardware store.
