Hi,
The rack is a Beto Child Seat Disc Rack. I wouldn't recommend it as it's a bit narrow but I have no option as I need it to carry the same child seat that my wife uses on her bike.
However, just google "disc rack pannier" and there are quite a few different sorts.
Some use built in extenders:
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Some replace the wheel skewer:
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And some - like the Beto, seem to just be wide enough to go over the calliper but need spacers.
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I've seen before - but can't find now, extended bolts (M5 size by the way) that bolt into the mounting hole and have very long heads that extend out and have M5 threaded holes in the end that you then bolt the rack to.
If I have no luck with the supplier sending the extenders (maybe they'll say 'that's how it comes from the manufacturer'). I'm going to hunt down the latter.
The only worry is that the leg of the rack - on this particular bike - passes right over the widest part of the calliper - so some spacers might still not be wide enough. In this case, a real fettling option - if you look at the previous picture of the rear disc - would be to bolt a flat L-shaped or curved extender onto the two mounting holes (that I've currently bolted the rack and the guard stay to) to provide another mounting hole to the rear that a such a rack might fix to (as the width wouldn't need to be so great).