We use small flat mirror mounting plates (normally one middle of the top of the frame and one middle of each side) but these do have the disadvantage of leaving screw holes if removed - we have painted bulkheads so easily repaired whereas varnished/oiled bulheads that is not so. I can't find a picture of them but they are just a flat plate with normally two holes which screw into the back of the picture frame and a tab overlapping out from behind the frame which one puts a screw through into the wall/bulkhead so are 100% secure.
svHyLyte mentions the risk of glass if used. Picture framers close to the commercial part of a port at least should be all set up to replace the glass with a rigid art quality rigid plastic sheet that serves the same purpose. I don't know what the plastic is but it is clear like glass (plastics of the normal kind, including "normal" acrylic, introduce a colour cast). It also reduces the weight to be hung.
Years ago when I first went into such a picture framer all prepared to explain what I wanted in that line he cut me short with "I know exactly what you want, we do it all the time for the ships and boats in the port" and it proved to be so
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