Of course this requires ignoring the AMA rules and possibly FAA suggestions as well depending on location, not that I support the AMA in any form or fashion. However, It might possible to do this in "FAA Uncontrolled Airspace" like my location.
The rules are different for all of us. Best to try to look up what applies to you and your locale. But if you want to give it a shot, just please do it in a safe manner so you don't put anyone at risk.
have heard Bruce reference 400 agl several times. if you havent already, check him out. he is both loved and respected throughout the rc community for his candor and approach to simply having fun. below is the link to his main channel page on youtube.
I twice went for a max duration flight using a sailplane equipped with a long range radio to allow smoother flights at higher altitude.Note this is only under certain conditions, and using a rough method of angles, trig, and timers we determined that the aircraft exceeded 1500 feet AGL, or 7500 feet MSL.
This is done with a spotter, as well as a "the plane is expendable" attitude. Aka, if anything appears on the horizon, the plane dives to ground level and lands. This is not, I realize, following the AMA suggestion that aircraft be kept below 400 feet AGL, but it was also not in the vicinity of an airport. The nearest is approximately 15 miles away.
It was fun, but after the first dive, I quit, and do not plan on trying a high altitude duration flight again.