Store owner replied:
Hi, sorry that it turned out to be no use to you, that is a genuinely annoying way to spend money.
the mount is made for the ERA nylon camera cage, not for a GoPro HERO adapter. That is stated on the product page, and you can also see it in the product photos, there is no GoPro-style mounting on it. It bolts straight onto our cage instead, which is what makes it as slim and crash resistant as it is.