quite a few solutions on ebay
https://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=p2380057.m570.l1313.TR0.TRC0.H0.X15mm+to+12mm+adapter+axle.TRS0&_nkw=15mm+to+12mm+adapter+axle&_sacat=0
This is the 15mm to QR I have:
Not sure how necessary the ridges are, but the adapter is directional so they must have some purpose.
The ridges should be matched to the locations of the sealed cartridge bearings in the hub (to provide support)
@hippy started
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quite a few solutions on ebay
https://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=p2380057.m570.l1313.TR0.TRC0.H0.X15mm+to+12mm+adapter+axle.TRS0&_nkw=15mm+to+12mm+adapter+axle&_sacat=0