Result:
I used the mount from a replica helmetlight, they fit perfectly. I just resold the fake helmetlight without the mount (the replica comes with that adapter and the helmet mount) with a little loss over what I paid... so the mount has cost me something like 5 bucks.
EDIT: As for the fake Mantas, AFAIK there's only a small production run of a resin kit for now, and it's quite expensive for a non-working replica... Element announced one as "coming soon" so it's not yet yet on the market.