To obtain the dimensions for the base, place the airplane, figures and other components on a sheet of paper, spaced as they will be in the diorama. Using a straightedge, pencil an outline that looks right. The border and sides of the base can be finished by gluing thin hardwood veneer to the plywood. Varnish or shellac your base to keep it from warping when applying groundwork, and to give it a nice appearance. Even plywood, nicely sanded and varnished can look quite attractive.
Concrete runways can be posterboard or cardboard with a rough texture.
You can paint the runway and allow it to dry. Next add oil spills and dirty wear areas for a natural look. Panel separation lines are then penciled lightly and tar caulking is applied. Use a gray color because black is too intense. Put some of the paint in a glue syringe and apply, using a straightedge as a guide.