My idea for a camera mount is not the least expensive, but is stable. (5 feet of pre punched angle iron is $14).
It would use pre-punched angle iron (available at the big orange box store). On the base of the scanner, set another 2x4 to put the camera "outside" the left side of the platen. Come up with 2 angle irons on each end of the base to form uprights. Come across the top of that with angle iron to finish a box. Use small corner braces to connect it all. Or if I'm really feeling good, then solder them together. Go across the middle of the box with another piece of punched angle for the camera to mount. This would allow the camera to move up, down, left and right. Angle iron is very structurally stable for the amount of metal used.
It could be done with wood for less money, but more labor. You would have to drill centered 1/4 inch holes, with 1/2 inch between up the uprights. For the cross piece in the middle , you would either drill holes, or you could route a 1/4 inch slot in the uprights and cross piece. Enough wood would have to be left for stability. With the slot method, I would glue a cheap level to it for ease of adjustment.