Was contemplating making one of these for a long while so I decided to bite the bullet and do it.
I started with a 1m length of 50mm PVC pipe, cut 150mm off one end. Glued a joiner to the short piece, drilled a hole and screwed in a brass hose spigot for the oxygen input. This made a removable end for the filter and oxygen input. This is just so we can remove the filter and input tube easily, otherwise you have to remove the whole stik from the wall to replace the filer. The filter is just a 50mm end cap with holes drilled in it, a piece of plastic from a "Goulburn Valley Pear Slices" container with holes drilled, and a felt dishwashing towel.
I bought a Oxygen regulator (Oxy Welding Style) on eBay new for $69.
Next I cut a pair of slots on opposite sides of the short piece about 3/4 of the way through. That is one on each side of the pipe. Then I did the same with 5 pair of slots, again on opposite sides of the pipe, of the long pipe. I used some 45mm plastic strip left over from some Holland blinds I fitted, but a strip of Perspex would be fine. Needs to be about the thickness of a circular saw blade. I used a 51mm hole saw and cut 12 semi circular pieces. like this
They fitted into the slots, then I used duct tape to tape and seal them. The stik looks like this
I spent about 30mins in Bunnings playing with plastic pipe fittings to work out how I could reduce the end of the 50mm pipe down to the size to fit the oxygen analyser and the input filter on the compressor. I had to cut off the T piece to enable the analyser input to be mid stream of the nitrox flow. Get some plastic tubing for the input to the compressor and the oxygen regulator and the stik is complete.
The analyser build documentation can be found here
Total Cost including analyser $220
Due to the high pressure that Oxy Welding uses and the low pressure that continuous blending requires, the welding regulator will not allow for granular adjusting of the output pressure. To resolve this a flow restrictor is required. Believe it or not, a hole so small is required drills are not small enough. What is required is the gap between threads of a screw.
Tap out the pipe inside the hose connector on the regulator 3/16 whitworths, then cut the head off a brass screw and cut a slot across it to form a grub screw, or you could use a brass grub screw. Screw it into the connector pipe and play with the amount of turns until you get the desired adjustments. Like this.