Flight instructor with student reported an NMAC while performing flight maneuvers in a popular practice area. Flight instructor performed evasive maneuver and checked for traffic.

Narrative

I was acting as a CFI training a commercial candidate over a lake popular for flight training maneuvers. We had traffic activated on iPads and a Garmin 430. We had completed clearing turns and had been over the lake for about 30 minutes with no traffic observed. After beginning eights around pylons at 2;500 MSL (to practice the basic fundamentals higher up); we had completed the first turn to the left and were going wings level prior to the second turn to the right. As the PIC turned wings level; a silver low wing warbird style plane passed below and off the nose about 300 ft. They made no maneuver to detour their route. We initiated a climb and started checking traffic monitoring devices. They were not visible on the iPads or Garmin; and no radio calls were heard. We had been monitoring approach in that area. We ended up leaving the practice area and returning to the airport. We contacted approach about the incident and they were unable to discern where the aircraft was or their altitude.In the future; using less popular training areas may help with traffic separation. It would also be nice to establish practice area frequencies for flight training communications.