I dont think there is an end cap solution, I looked into it when I couldnt find the 12mm SP hub in stock ANYWHERE ..
The adapter I have that takes 15mm down to QR is a specially machined tube arrangement. I wonder if using a straight bore tube will cause issues.
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
@hippy started
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I dont think there is an end cap solution, I looked into it when I couldnt find the 12mm SP hub in stock ANYWHERE ..