As far as I know, every liferaft manufacturer makes a rail mount holder to fit the life rafts that they have packaged for exterior exposure. Have you contacted them? Design of these isn’t always simple, especially if you use a hydrostatic release.
Our thinking on life rafts has slowly evolved as we have lived with one, and reading about those few cases where people actually needed to deploy one.
On an Amel, with multiple watertight compartments, it seems the most likely reason for needing to take to the raft is an uncontrolled fire on board. With this in mind, I do not consider storage below deck a good idea.
I have no issue with storage in the bottom of the Amel deep Port side deck locker. Anything stored on top would just be thrown overboard in an emergency. Yes, our four-man raft is heavy (80lbs/40kg) but not so heavy any single crew member could not lift it if they HAD to. A 6 or 8 man raft changes that calculation a lot. A 8 man raft is really heavy and awkward. If I felt the need for 8 man raft capacity I would seriously consider using two four man rafts!
We have ended up with our valise packed raft in the cockpit locker as part of our ongoing effort to move weight as far starboard as possible. I am liking it there for another reason, it gets looked at a lot! It anything was going wrong, we would see it.