Need help with materials for a 6.5x8 layout than can be moved

I was also thinking about modifying this idea instead of foam using a 1/4 inch plywood instead.

See a light weight layout here:
Portable Layout for Train Meet Display
Details and Construction Information

This layout was built of 3/4" Gatorfoam.  For your larger area use 1" Gatorfoam.
See Modeling with Gatorfoam

Gatorfoam is a unique, strong, laminated panel with a core of extruded polystyrene and a face made of resin impregnated wood fiber veneer commonly known as Luxell.

Interesting stuff thanks!

You might consider hollow-core interior doors.  They are very light and vey rigid.  My current layout is about 8'x10' and comprised of four doors screwed together.  I cut them up a bit and made some other mods but they would work fine just as-is if the sizing fits your application.  While mine is supported off the walls of the room on angle brackets, the assembly is rigid enough to pick up whole.  The doors are available from any of the big box hardware stores and come pre-primed.  You want the ones without any raised panels.  They are 80" long and available in widths from about 2' to 3'.  As I recall they were in the neighborhood of $25-$30 each.  I screwed the doors together using Kreg pocket hole screws.  They do resonate the sound of moving trains a bit, so they aren't the quietest, but this is the second layout I have done with them and overall I like the approach very much.  If you're not familiar with Kreg joinery, just Google it.  You need a cheap jig and a box of their screws and you can screw these things together lightning quick.

I don't see any big gain by doing that. 

Changing the track plan would result in less trains running.

I would place some tabs or brackets on the inside of what you have and drop in a pre-assembled piece of insulation foam or gatorboard for a scene.

Watching the video I saw a Disney looking castle in the center with four bridges going to it. leaving big gaps like the castle is floating over the pond.Then, turn on the pond lights and have accent LED's shining at the castle. Perhaps North Pole Santa's building. leaving the hole open with bottom up lighting would create a nice fantasy scene.

Whole table:

Figure 8 in center with loop around - crossover switches to outer loop - automatic switch control to keep changing the routes - I think that they are "scenarios" in DCS. Perhaps three trains on two loops. The HO would have to be relocated.

That may worth covering the pond. But, it is trains only.