For mag clamp, I used a MAGCINCH (brownells) for an AR10. Minor filing needed.
It holds the mags nice, but shotgun recoil knocks the shells of the second mag right out of the mag. Plus I found the coupler caused mag issues. YMMV.
For pouches, any .308 mag pouch with an open top should work well enough, I would advise you to start your search there. m14, ar10, FAL etc.