I've seen them hung the way you normally would, then velcro attached to
the bottom of the frame and wall. Or just use velcro on the whole
frame, but avoid frames with glass. They make some velcro today that
really holds on to anything.

I guess it all depends on the application. If your running offshore in
some good seas, screw through the frame into the wall. But if your a
bay guy that thinks 12 knots is flying, anything will work.