Circ Saw works just fine if the plunge cut is done right. Even 1/4" plywood would be strong enough to prevent racking. l Use it all the time as a back to stop racking and to square up the carcass.If you used a single sheet of plywood for the front face (on top of the 2x4s), that would provide lots of lateral strength (as well as vertical strength) - as long as it was thick enough. So you could probably do away with the middle two (2) legs in that sort of design. But I would probably use a different sort of tool than a circular saw to cut out the holes in the plywood.