To which Simon are you referring?
I made up a couple of roller gauges, which got passed on to others: to get a small batch done would cost rather a lot of money, due to the setting up time for an automated lathe, so unless a hundred or so were required, it could be cost prohibitive.
They are a simple turning job, providing location for the rails but not clamping them or holding them in any way, i.e. there is a 1.5mm circle sticking out. But there is a clever twist to them, and the ScaleOne32 track standards, as you will see...
So, a piece of rod is put in the 3 jaw chuck (or held between centres if you are properly trained): you need about 60mm plus chucking length. Diameter is unimportant, but you want to be able to find it. You may wish to put a small dimple in one end and put a rotating live centre in the tailstock to hold things true and steady - you will know your own lathe better than I, and whether this is required.
Make a smooth cut to get everything to the same diameter - if you have power feed, this is the time to use it.
The best tool to use for the rest of this is a parting tool, or one ground to have a narrow but square end.
You now need to reduce the diameter by a couple of millimetres for 3mm from the tailstock end: these dimensions are not critical, but you need to stick to them when you make the cuts. Pull back the tool and advance it towards the chuck. You need to cut into the metal and leave a 1.5mm wide circular prong, to locate one of the running rails and checkrails. Remove metal to a depth of 2mm (or whatever you cut to) and a distance of 42mm. Pull the tool out again, and leave another 1.5mm prong, then advance again and cut for 3mm plus the width of your tool, then part off the roller gauge but leave the rest of the metal in the chuck.
Now for the clever bit...
Measure your roller gauge to find the smaller diameter and bore out the stub in the chuck to this diameter: you want a sliding fit, but no slop. Then part off a ring 0.5mm wide.
You have now made your gauge.
It is used "plain" for straight track and curves above 3m/10' radius. Below this, you pop the washer onto one side to widen the gauge. You also use this wider (2mm) dimension to set any check rails on this gauge - check rails are set to the check gauge from the opposite running face.
Simon and I have pondered whether or not the gauge widening should be applied below 15'/5m, but have not come to any firm conclusions yet.
Since I need to make one of these for myself, I will take photos as I go along. To those who say they cannot afford a lathe, I would say that you can: a small lathe such as a Sieg C0 is a couple of hundred pounds new - with accessories closer to ?300, I expect. I picked up a Myford ML10 for ?400 last October, funded by selling some unwanted stuff on Ebay. This is less than the cost of most 0 gauge kits, and frees you up to sharpen as many pencils as you would like, or if you buy Peter Wright's book
"Model Engineering: a Foundation Course" you can learn how to use it and other tools properly.
Actually, you should buy the book and read it anyway: terrific value.