Create runway for a mission

HiI am creating a mission in a wild place with no near airports. I would like to create a rural runway as a mission object and put it in the place I want. I dont want a real airport but a single flat place with no autogen obstacles and a single strip grass or gravel runway. Default scenery has any flat zones too small for a secure landing. How can I do it?Thanks in advance.Raul de Frutos