There is no wire attached to my plug cap. The plug is an NGK BPMR6F. The cap is pushed on as hard as possible & clicks on the plug top. There is a rubber insert in the plug cap that centralises the cap on the plug. With the cap in place, there is a gap of about 1/8th" around the hex. It does not look possible to close this gap easily without distorting the cap badly.
I think the best solution is to solder a wire to the plug metal braid, with the added security of a couple of cable ties around the joint & ground that.