I got both the SS tubing and compression coupling from McMaster-Carr. OnLine Metals is less expensive net source for SS tubing and they have it in several alloys (e.g. T-304, 1/4" OD x 0.035" wall is $1.68/foot). The local Ace Hardware carries the 1/8" pipe tap (about $5). If the one near you doesn't, they can order it for you.
The male, threaded, end of the compression fitting must be drilled out to allow the 1/4" tubing to pass thru it. Drill the fitting out from the compression end of the fitting so as to help keep the hole centered. SS is a bit difficult to drill so, a drill press, a vise (or maybe vise-grips) to hold the fitting, and lots of cutting oil are recommended. I think chucking the fititng in a vise and attacking with a portable drill is doable although I've not tried it. If the hole is a little tight for the SS tubing to pass through, expand it with a round file. (Tip: Slide fitting onto file, chuck outboard end of the file in a vise, grab other end of file and file by moving the fitting while applying pressure towards the surface being filed on the cutting stroke.)
The compression fitting screws into a tapped hole in the keg. Make sure the hole is not too big- the end of the tap should just barely fit in it. DO NOT OVER-TAP the hole in the keg (i.e. tap the hole too deep)! Keep the tap straight as the hole is being tapped. Use cutting oil while tapping the keg and back-out and clear the chips often. Install the fitting using teflon tape. Don't overtighten- 2-5 ft.-lbs of torque is all it takes.
Discard the SS compression seal that comes with the fitting. Use a nylon that's one intended for brass fittings instead. This will allow for low tightning torque and, in case you tighten too much, will prevent the seal from seating into the tubing. If this happens, the dip tube won't be as easily removable!
The end of the SS dip tube was plugged with a bit of SS rod pounded into the end. A leak tight joint isn't needed. The kerfs in the business end of the dip tube were made with a Dremel and a abrasive cut-off wheel. The exposed portions of the kerfs were sanded then polished on a buffer to aid cleaning by minimizing hops clinging to them. The tube gets passed thru the compression fitting from the inside.
The 3/4" x 3/4" aluminum angle provides support for the vinyl tubing and as well as a place for placing volume marks.
Update / Warning:
Hops can plug the kerfs in the end dip tube thing! I had it happen once after I interrupted the boil and restarted. Like a dummy, I boiled it down to 4.5 gals. before realizing the problem- DUH! I now have a length of SS mesh like that used in the maniold attached to the end of the dip tube. Another alternatives are a SS scrubbing pad on the business end or periodically blowing gently into the free end of the vinyl tubing to clear any hops.
The 3/16" ID heavy wall vinyl tubing is cut long- about 2.5' long. This allows for surplus at the bottom end for forming a "U" which functions as a siphon when the wort level in the keg is low and surplus at the top for purging air (more below). Here is info on securing vinyl tubing to the end of the SS dip tube.
The vinyl tube is only temporally fastened to the upper portion of the angle- it needs to be easily removable ("why" is below). A ubiquitious twist-tie is shown in the drawing above and works fine, but a nylon cable clamp is a less half-assed in appearance and pretty easy to adapt for this use. Here is how it's done.
Calibrate the gauge- FWIW, I poured in a measured gallons of luke warm water and marked the resulting level on the alumimum angle with a pencil. Be sure to purge the air from the gauge (see tips on using the gauge below). I mostly eyeballed the marks for the 1/2 and 1/4 gallon ticks.
After installation and before the first use in brewing, boil some water in the keg. Check for leaks at the fitting. Flow a gallon or so of the boiling water through the sight glass- just unfasten tubing and lower the free end into a container. The tubing will get hot and soft, so watch for kinks. This is also am excellent time to practice sanitizing the gauge (see below). Flow off a sample for testing. After the the sample cools, taste and smell it to ensure there's no plastic odor or smell. I've never had this problem, but, it's best to be safe and not risk maybe "plasticizing" a batch of beer.
SANITATION Sanitizing the gauge before boiling won't work- the wort in the gauge during the boil will likely be too cool to even pasturize it much less sanitize the tubing. Here's what I do: