Here, I am sketching-in some construction ideas. I like using seven 2" thick 4x8 foam sheets (which if you have not used, are stronger than you might imagine). They are easy to trim for the varying shape of the layout surface. They cover the surface pretty well, needing only a shelf for the NS staging. Because access and reach are quite good, I will not be up on or leaning on top of the foam to reach to the back, so the foam sheets are strong enough.
The layout surface will be supported by wood tables (design not certain yet) to give a height of 46-48". The foam sheets are strong enough for 16" centers between runners. Because I am lazy 
I will likely just lay-down and glue Woodland Scenics grass mats to start, and then build up the elevations, grades, and etc. on top of this surface. The stream on the left below grade will need some additional thought - it will be 2" below grade, so a length of foam below the 2" thick foam underneath will serve.
This construction will make tear-down easier after I am gone and the space will need to be re-purposed, perhaps in prep for a sale of the property.
