have two special materials added to my max materials library, called Default and Default2:
the easiest way to create these is to just import a navmesh, then use the 'get material' eyedropper on it. set the colours as shown to make it easier to understand which material is which, and i have mine set to 50% transparency so that overlaps etc. are easier to spot. the Default2 material is identical except for the colour-shift; i use that one for preopt and manifold objects, so i can have these and the navmesh both visible at the same time and still tell which object is which. Every time i import a new navmesh, i get these materials from the library and apply Default to both the Infantry/Vehicle meshes and go on from there.