Nice one
Have done a few similar top cap adaptors to that in the past, usually just tap an M6 thread up the centre then hammer a 5mm allen key into it - it broaches a short (few mm) allen key fitting into the thread so you can fit the adaptor, then screw the top cap as normal into that thread.
Looks like you have done M18 x 1.0 (which is the correct thread size), just thought I'd pop that in for search-friendlyness...