Since the top of the gun doesn't move, mount it to the top. You can't mount it up top on regular autos because of the slide movement. You could drill/tap. Or, better still, incorporate a base that uses the front and rear slide dovetails, after removing the regular sights. Make little "blocks" that fit the dovetails and are drilled and tapped. Mount some kind of rail to the blocks. Mount the optics on the rail. Just a thought.