In such case i'd better make bracket for Alixpress 19 mm slave cylinder, or modify it. At the end, i can make drawing and mill the own slave cylinder))) I think it will be cheaper and faster.I really doubt you will find the correct compatible seal. They are not just regular "o-rings" but special shapes specifically designed for the particular piston, see photo below. You can technically use DOT5 silicone based brake fluid as it's compatible with both the mineral and glycol compliant seals, but DOT5 has terrible lubrications properties, and it will destroy your magura slave in a few weeks of use. Also the clutch travel will not be smooth and bind up in it's travel.
Good luck to you, just don't waste too much money as I did...
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The end will be in month, two.