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Woylie said.....I haven't really seen anything mounted on rails on bigger boats, if anyone has any photos I would be keen to see.
OK, it took a bit of searching in this computer, but here are some solutions:

The first one is my yacht, and I took the outboard and cut a huge hole in the yacht to make a well. The stern lamp has a welded plate on the stern rail, but there is plywood there because the holes don't line up/ too hard to drill, etc. That's why the white lamp is lower than the steel plate.
The 2nd photo is much neater. If you can't get a small plate of stainless steel welded to the rail, just use plywood like the white big square plate used to mount the dinghy outboard motor.
The 3rd photo shows things much clearer, and it depends where your OB is mounted. Hard to steer from that position though.
And the 4th photo is a timber bracket you could make, perhaps you could mount it off-centre too.
To mount a small outboard motor, or a timber bracket like above, you can use rail clamps or tube clamps to the timber or plywood:
www.whitworths.com.au/rail-clamp-small
I hope this helps.