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@Max
Hi Tommy - this is a cut and paste of a post I made on other forums about this, bear in mind this all relates to an MTB with different O.L.D.
Just done a couple of hours riding off road fixed in Epping Forest this morning BTW, great fun. But - I had forgotten the disc front wheels are also dished so I will probably scrap the idea for a street fixie - off road the flip flop cassette/disc mont works well as it makes a great do anything wheel - SS (with infinite chainline adjustment), 7/8/9 speed cassette, custom 2 or 3 cog set up, anything really. However, on road with a sleek fixie I think proper fixed/fixed non dished conventional wheels look much better.
Anyway, back to the 'how-to':
I used a cheap pressed steel cog that came with a SS converter kit. These are quite easy to drill. Mine is an 18 tooth. You need a standard 6 bolt dics to mark it up - lay the disc on top of the cog and center it up - make sure of this - eye up the centre edges of the cog against the centre circle of the disc and measure the distance in three places to check it is in the middle. Hold tight and draw the 6 round circles where the bolts go. Centre punch these and then using good quality HSS bits drill the holes. 2mm first all the way through and then a 5.5mm to finish. De-burr with a 7mm or similar and that's it. Mine ended up spot on - just take some time and care - worth a try for a few £'s specially if it is not for you. The washers are just to perfect the chainline.
A pressed steel cog is easy to drill and at around £3 each if you screw it up it doesn't matter - try another. The cog I drilled sits more central than the Surly cog on my cassette hub. A proper hardened track cog would be more tricky without a decent pillar drill.
Regarding chainline, the disc mount position is obviously fixed in the ISO dimension. It can be spaced out a little simply by fitting washers between the cog and hub. If mounted directly to the hub the chainline in mm will be around 52/53mm IIRC. This is easy to check on your bike if you have a disc hub - measure from the inside face of the drop-out to the centre of the mounted disc. Subtract this figure from 67.5mm and that is your rear chainline. Check this with your current front chainline and you can see where you stand (measure from the centre of the seat tube to the centre of the teeth). You can fine tune the front chainline by varying the BB/spindle length/chainring position etc.
Couple more pics here, just about the most solid way to mount a cog, far better than any lockring or similar system: