As a general answer - remote locations in narrow deep valleys are potential candidates. Factors that influence suitability include proximity to 'civilisation', major cities, airports of any sort and air routes. I live in New Zealand. Much of the southern part of our country consists of extremely rugged and mountainous terrain and I am certain that numerous areas would meet your requirement - but it would take research to establish which are most liable to.
Once you found a NZ area that met your specification apart from random unfortunate incidents it may even be possible to obtain the cooperation of local aircraft operators to stay out of a very tightly defined area. You'd no doubt need to satisfy those concerned that you did not have ulterior motives.
You specifically mentioned chemtrails. You may have to take special security precautions to hide the plan from the chemtrail establishment and it seems logical that anyone in the chemtrail generation business would want to crash any ;party' that seeks to exclude them. This applies for any site you choose.
The Chatham Islands are almost 1000 km to the East of the New Zealand mainland.
There is an airport used by flights to/from the mainland (located on the tongue of land between the upper and lower lagoons approximately central to the island. There will be locations on the island where the topography and location would make mainland - Chathams flights inevident.
The extremely remote and bleak "Auckland Islands" are located almost 1000 km to the south of the NZ mainland. There is no airport. Helicopters occasionally visit - almost exclusively on rescue missions when scientific parties 'get into trouble' - as happens occasionally - when the helicopter flights are rare enough to be news worthy. (One was lost near the islands recently and the crew of 3 were extremely lucky (as well as skillful) to survive).
If more ruggedness and isolation is required the further still to the South-East and unplesantly closer to Antarctica Campbell Island would almost certainly 'fill the bill".