so this can be a "proof it can be done", here's two pics of my a-d 91xg mounted using a j-mount (a-d stm-1000). i mounted it last oct-nov (2012); it survived the 2012-13 winter and some pretty strong winds we had recently.
i would add that a proper base / anchoring of the j-mount is essential to keep the whole assembly in place. in my case, it's 5 2x4s forming a block between two roof trusses -- see 3rd picture. (the four arrows point to the holes for the lag bolts.)
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