The cap covers the hex on the plug, it looks as if it was made for a larger size plug. The rubber insert in the cap extends almost to the end of the cap & the only way a hose clip will work is if some of the rubber was removed. Even then I would have to cut slots up the cap side to avoid distorting the cap to much.
I think the BPMR6A plug would fit the cap properly.
Many thanks for your help to a gasser newbie.