C172 Flight Instructor reported a near miss with an aircraft that was performing aerobatic maneuvers outside of the designated aerobatic area. Instructor made an evasive maneuver to avoid a collision with the other aircraft.

Narrative

While conducting slow flight with a student 3500' MSL and after executing clearing turns; a Christen Eagle II aerobatic biplane traversed the practice area at 3000' en route to the local aerobatic box south and west of ZZZ airport. We received a traffic alert from our ADSB-in and turned to avoid the traffic. As the aircraft passed in front of us with approximately 2 miles of separation; it executed a sharp right climbing banked turn in what appeared to be the first half of a chandelle or lazy-eight. This maneuver put the aircraft on a direct collision course at our altitude and opposite bearing. I took control of the airplane and executed a banking steep descent away from the oncoming aircraft; but because we were in slow flight; we lacked initial control effectiveness to quickly change course or altitude. I estimate that the biplane passed within 200 lateral feet of us at our altitude before executing another sharp turn away from us and beginning a vertical loop. I believe the primary cause of the incident to be the aerobatic pilot commencing aerobatic maneuvers outside of the designated aerobatic box and in a busy practice area and IFR approach corridor; possibly without adequate traffic awareness or clearing the area prior to maneuvers. A secondary cause of the incident was our slow speed and high pitch attitude; which limited our forward visibility and ability to quickly react to traffic conflicts. Two factors that could possibly reduce the risk of this type of incident: (1) establish a practice area common traffic advisory frequency that is shared with aircraft operating in the offshore aerobatic box; and (2) remind local pilots of the location of common practice areas and aerobatic boxes.