A pilot practicing acrobatic maneuvers reported a NMAC with an aircraft in level flight.

Narrative

We were in the Salinas Valley doing airwork. We cleared the airspace looking for traffic; once we both concluded the airspace was clear began a four maneuver series at 5;000' MSL. The first maneuver was an Immelmann into a Split S that led into a loop ending with an aileron roll. Immediately after rolling wings level I began clearing all quadrants of the airspace around our aircraft beginning with my left rear; as soon as I got to the right rear window looking at the 45 degree behind I saw a low wing aircraft; it filled up the window; wing tip to window corner. It was at the same altitude turning into us. It was close enough that I could tell the type of landing light and I could see the individual vanes on the air intake. I immediately took evasive maneuvers performing a descending right 360. The other aircraft continued along to the other side of the valley. Needless to say we kept a close eye on that aircraft while we proceeded inbound to our home airport. After some time processing the event I went to 'Flight Radar' and reviewed the playback of the airspace and was able to determine the time of the near miss and the other aircraft was an Aircraft Y. I believe what could help prevent near misses such as these from occurring would to have a dedicated training area that is clearly defined in the Chart Supplement; and possibly on ForeFlight like Notams/Drotams; and TFR's. Maybe aerobatic pilots could reserve blocks of airspace? Part of aerobatics is flying in a defined box; so that airspace could be reserved.